Life After People/WMG

Life After People takes place in the same universe as Firefly.
We all just left, explaining the lack of carnage.
 * Also this theory can also be used/shared for the Animal Planet series The Future Is Wild in where Humanity leaves the earth after a 2nd Ice Age.
 * If we all just left, we left in a hurry, leaving everything running. Kind of like the whole planet sharing the fate of the Mary Celeste...

Life After People was filmed by time travellers.
It was listed as a documentary. But it is documenting events that have not happened yet. QED.
 * Cue obligatory Time Lord reference.
 * The History Channel is run by Time Lords? That's...quite convenient actually.

Life After People was filmed by aliens
At times there are small camera effects that appear that someone (or something) is filming the show as if it were an actual live taping. And since the series at nauseum constantly insists that no humans survived the only logical conclusion is the entire show is filmed and produced by aliens who arrived shortly after The End perhaps to originally make a standard documentary about Humans. However after reaching the planet they discovered Humanity died out years ago from unknown causes and just decided to film the remains.

A couple of months later they give the footage to Network execs who give it the greenlight and it becomes one of the most successful series on "their" version of the History Channel.

The people of Earth are gone because of the diabolical plans of Stanley from A Troll in Central Park.
He wanted to see a world that was 'absolutely green'. What we see at the end of his movie is the beginning of a Life After People.
 * ........oh god.

Humanity disappears in the year 2012
The Mayans were half right. The world survives, but humanity does not.
 * Alternately, because the 2012 Mayan prophecy is actually supposed to be a prophecy of 'new beginnings' rather than the end of the world, it's actually a new beginning for the rest of the planet, not for humans. Kind of implies that Humans Are the Real Monsters though.

Humanity has been abducted by the Reapers
In an alternate history of the Mass Effect universe, the Reapers invade the galaxy before humanity discovers the Mass Relays. Humanity is destroyed alongside other sapient races, while the rest of planet Earth remains untouched.
 * Then shouldn't the world be wartorn?

The machines have risen!
A sentient artificial intelligence (or group of AI's) has wiped out humanity, and is recording the decay of the civilization humanity left behind. Unfortunately the AI does not have sufficient capacity to spare every computer on the planet (or it deems non-AI machines as 'inferior' and not worthy of salvation).

The Irkens had something to do with it
Doesn't seem a bit dark and suspicious?