Girls und Panzer/WMG

The Girls und Panzer Universe's Economy and Society
The Girls und Panzer universe (or Japan at least) appears to have loads of resources to throw around for purposes that would seem frivolous in our world -- see the converted super-carriers used as schools, the deployment of large numbers of tanks for a high-school sport, and the casual emptying, destruction, and reconstruction of towns to provide those tanks with play arenas. (Not every battle takes place in a town, but Oarai's first match, against St. Gloriana, was a friendly and was held in town, so clearly it's not unusual.) What's more, Japan considers it a matter of national policy to promote tankery, and considers a school's tankery performance as a critical factor in funding decisions. Families with strong tankery histories own Big Fancy Houses and hold great sway in these matters as well. This could imply one or more of these variously fantastic and terrifying scenarios.

Post-Scarcity
Perhaps the GuP world has achieved post-scarcity, and can easily afford to build tanks and supercarriers if they feel like it. The major weakness of this hypothesis is that, even in a post-scarcity economy where resources are plentiful, land is still not easy to duplicate, so the enormous Nishizumi and Isuzu estates still need explaining. (Perhaps these families make their money in some other way, such as ownership of mineral wealth that is necessary to build tanks?)

Post-War Boom
Less lightheartedly, Japan may be benefiting from a massive military-industrial complex. As the United States did in our world, perhaps they emerged victorious and largely unscathed from a world war (or maybe even the World War II) and are taking advantage of the built-up industry with tankery.

Still At War
Even less lighthearted is the possibility that this war is still ongoing (perhaps nuclear weapons have not been invented, and some other geopolitical changes occurred as well, such as a German victory or near-victory against Britain or Russia, or the fragmentation of the Allied and Axis powers, so the war has stalemated). In this world, the government uses tankery as a pipeline to produce talented commanders and tank operators, and likewise uses the school-ship concept to produce a strong base of navy personnel. The promotion of tankery prowess as being an essential element of Japanese womanhood and the recompensation for property damaged in tankery matches is all part of the propaganda. (This gives yet another spin on Shiho Nishizumi's behavior towards her daughters -- in this world, she would certainly have been used on the front lines for her tankery prowess, and may still harbor lingering trauma. Combine that with the nationalism necessarily enforced by this scheme, and she may have concluded that the best way to bring up her children in this world is to forge them into undefeatable commanders...by any means.  Abusive, perhaps, but it might be the only way she can think of to guarantee their survival through the war.)  Certain schools are groomed to adopt the cultures, practices, and weapons of the various nations involved to pre-train commanders for battle with or against them. As a flexible commander who is able to put together a strong army out of rookies, devise strategies and emerge victorious against opponents from every 'nation', and then turn around, forge an alliance out of all of those nations at once, and defeat an elite university team, Miho will surely be a prized asset when she graduates. She may be in for some heartbreak...

(This would also lend some Harshness in Hindsight to Kay's comment that "this is tankery, not war", and the relative lack of men in the cast.)

Simulation
The tank battles are actually all in virtual reality, and tankery is simply a Japanese national obsession akin to football or soccer in other parts of the world. (Possibly subtly encouraged by the Japanese government, to have a pipeline of talented soldiers in the event of a war.) The immense cultural and capital influence wielded by the tankery ecosystem (including the Nishizumi family) is not unlike that of the NFL, NCAA, and (especially in e.g. Texas) high school football systems in the US.

This is why no one gets broken bones or worse when their tanks are flipped upside down and set on fire. The girls Miho rescued from drowning in her character-defining moment were in no real danger at all. This makes her mother — and Erika — not quite as dickish as they'd seemed.

On the other hand, the simulation is so good that people, especially young people, often lose track of the fact that this isn't the real world. That's why Mako, flaking out, thought she had to swim to get to her ailing grandmother, rather than simply logging out of the VR and catching a taxi, and why Miho cares so deeply about protecting her team members. Even if they won't actually die, the sensation of drowning or exploding may be just as traumatic as the real thing (we see that Miho has recurring nightmares about that mission). Shiho might dismiss this as being too touchy-feely (perhaps the simulations in her day were of lesser quality, making it easier to stay detached), but as Maho pointed out, Miho's determination not to yield her subordinates even to simulated death breeds an esprit de corps and devotion to their leader that helps a pack of newbies defeat veteran players and overwhelming odds ... so this hypothesis does NOT mean Miho's the idiot Shiho and Erika call her. Rather, she's deliberately playing at a higher level of difficulty than they are. And beating the odds anyway.

Virtual reality also mitigates the apparent wastefulness of the whole program. That shopkeeper wasn't talking about remodeling with insurance money after his shop was wrecked, but remodeling with profits from people visiting the shop because it featured so prominently in the broadcast. Free advertising, yay! And those impossibly large ships? Hey, the computer simulates those, too. Everything's really all taking place on land at Oarai.

Some side interactions complicate this hypothesis by depicting the use of tanks in ways that would be completely nonsensical in even the most thorough virtual arena, such as waking up Mako with a blank round (unless she's so narcoleptic that she falls asleep in the virtual world?), going shopping, being repaired by high-school girls, or being transported away from Oarai in the face of its impending shutdown. However, it may be the case that the tanks themselves are real (presumably they are permitted to be driven around in the real world for publicity purposes) and are placed in simulator pits for battle, much as modern aircraft simulators use replica cockpits mounted on hydraulic platforms and surrounded by screens. Having the real hardware would enhance the sensations of the simulated battle, and would also explain why Team Anteater struggled to adapt to tankery from their gaming experience (as the tanks would still require physical effort to operate) and why the automotive club was given the temperamental Tiger (P) (as any real-world mechanical malfunctions would translate into the virtual world).