(sorry if you misunterstood anything, english is not my native language though I think I'm pretty fluent at it)
Because making a Grand Theft Auto game is hard. You see, there's this thing called 171 which is a GTA-like game being developed, but the difference is that its map is on a brazilian city. Betagames Group (the independent company behind this game) has gattered more than 10k dollars through Catarse, used lots of models from other sites (which I hope they credit them to) and it has been in active development since 2015 but its pre-alpha has released just recently.
I can't say the same for linear games though since there are some kinda nice games released, especially those based on Source engine (e.g. Portal TWTM and Hunt Down th- no, this one doesn't deserve to be called a release).