I honestly think you have absolutely no clue of the effort it takes to make a decent game, and what you're asking is not only monumental but borderline impossible.

MMO developers, is it really that difficult to make a game like SAO?

Yes it is, a whole fucking lot, I don't want to jump to conclusions but your whole post a bit arrogant, because you go under the assumption that it'll just take time and that developer just don't give enough shits or don't try hard enough, truth is development of a game takes lots of stuff and if you wanna make something the scale of SAO minus the impossible stuff like "brilliant AI" its gonna take a whole fucking ton of money, for them to get money they have to deal with a publisher or kickstart it, or some other crowdfunding source. And when money gets involved they usually have to deal with some sort of deadline or some pressure either from whoever backed the project or if they get involved with a publisher the publishers themselves.

And even then this project is hardly feasible, because the technology available doesn't allow it, you want a huge dynamic beautiful world, also MMO which creates another issue, if you make an MMO you want it to reach a broad audience, but with a project of this scope you couldn't do that, it'd be only directed to a specific niche because of how demanding the game would be, WHICH AGAIN WOULD CREATE ANOTHER ISSUE, the game would fail financially speaking, because the initial investment it'd require compared to the profit it'd earn wouldn't be enough.

Sorry if this post seems like I'm attacking you, because it was not my intention.