Because it's a legitimately fun and good game.
People expect everyone to rip into it because the main story is only like 2 hours long which is kind of funny considering nobody had a problem with Gone Home winning GOTY and it being $20 and only being about 2 hours long with almost 0 re-playability where as this not only has a 2 hour campaign that has re-playability it also has side missions.
But, it's the internet.
A lot of people on GAF had a problem with the price they were charging for Gone Home. I remember seeing plenty of posts commenting on the fact it was $19.99.
It's all about perspective too.
Gone Home, a game at least provided a complete experience and story even if it was short, developed by a small indie team as their debut game, with some pretty serious life messages contained in it too.
vs
Ground Zeroes, which appears to be just Konami, a not small publisher, cutting out the opening act of MGSV a la Tanker/Virtuous Mission, to sell as a $30 product in order to wring out as much money from their fanbase.
Who looks like the bigger bad guy here?
------------------------------
For funsies and because I'm in a Metal Gear mood with the release of this game I read through Jeff's original review of MGS for the PS1.
http://www.gamespot.com/reviews/metal-gear-solid-review/1900-2546002/
He gives the game a great score but knocks it for its "insultingly short length".