Yeah, I hear you. That was the path Chaosium took. Every time they wrote a new RPG they just tweaked the same basic core rules, and then eventually extracted them and called it BRP. TSR interestingly avoided that path entirely, though I cannot say either company was more or less successful.
Anyway, it is just hard to build a lot of games on D&D, which assumes a very strong power curve where you start weak (almost normal human) and then ramp up to almost godlike. It works well for the genre, and maybe for GW, but imagine Call of Cthulhu where your PCs are 12th level and slaying Shuggoths and whatnot? It is definitely NOT what people play that genre to get! Or a supers game, nobody wants to start their supers game as a normal human, they're super heroes, from day one. At best they might play an 'origin story' or something. So we see that D&D, as a framework, is not all that applicable to these games.
This is also why 5e will never be extended into this sort of 'universal system'. WotC tried it with d20 Modern. The rules WORK, it is certainly quite feasible to build the mechanics, but the tone and other aspects that d20 Modern promotes are not really suitable for a lot of games. Or you have to get rid of a lot of the D&Disms, like levels (or many of their effects anyway) at which point one must question the value of the entire enterprise. So, I could imagine a
Oh, yeah, they have a lot of talent. It has been a LONG time though since WotC seemed interested in any game except D&D though. They allowed their Star Wars license to lapse and Saga was the last non-D&D RPG WotC has tried, unless you count the 4e version of GW (I guess that does count, though it was not a very ambitious effort overall). Honestly, since acquiring TSR WotC has really not done much outside of D&D. They put out d20 Modern, but didn't do a ton with it, and that was way back when at this point. Prior to that you have to go all the way back to Everway, which was a pretty interesting game in some ways but died pretty quickly.
It would be interesting to see what they could do, but I'd be very surprised if they put their own people on anything like that. I could see them perhaps farming out a property to a 3PP to do an 'official' version of something or other. Beyond that they have seemed pretty content to leave the experimenting to others.