The most reason is no profit from it.
A good game must combine with good graphic, good gameplay, good music, good story e.t.c
But most of game can't get everything, because it require a whole team to make and you much pay for the staff, that why they must lower some parts to acceptable. A good story game might have bad graphic or simple gameplay, a good graphic game might have shit story.
If you make a perfect game, you must sell it at high price, but not mean everyone think your game is good (look at AI shojo and it's cost, good quality, but stupid AI and boring story), not many people willing to pay and they'll looking for free download instead.
So, ask yourself, will you spend money, time, and effort to make a good game and then let everyone play it for free?