Why? Developers choose what they offer on there, it's not like they are strong-armed into anything (outside of MS first-party that is).
Assuming that they are doing some kind of Deezer style revenue share (as mentioned in another thread), the potential return for developers goes up exponentially as the number of subscribers increase. Sony and Nintendo first-party are the exceptions more than the rule, most games (even a lot of the good ones) will be lucky to sell 2 or 3 million units in the standard retail market (with many of those not being full price sales). We've seen what the existing market will give us, quality from the studios built to sell hardware and safe and very predictable output from third-parties.
I'm willing to sit back and give a new model a chance if it lets developers take a chance on something they would have been afraid to green-light otherwise.
Spiderman, for example, is a solid game that works in the standard model perfectly, but could also be perfect in a GaaS type situation. That game could easily live on forever with dlc expansion, basically adding a new comic book to the game with the addition of new villains and plot points. Maybe a few new interior locations, and some tweaks to the city for the new story. GaaS != low quality, though it could, it all depends on who is developing it.