Because It Makes It Easier For Fanboys To Wank
And this is just as fucking awful as Ultimate Extinction was. It was a half-assed and lame idea when Ellis shat it out(an example of what his work looks like when he's doing it for the dollar, or specifically in that case, to fill in, I think, for Mark Millar, who was ill at the time), and it's just as lame now. That, I think, was the point at which the changes done in the Ultimate universe started to seem stupid and pointless.
I remember the idea had been to refit them to be ready for the movies. But consider their greatest recent success, Iron Man, apart from period- based changes is pretty much the Stark we know, not the one in the MUU (and after reading Orson Scott Card's UIM, I never want to see that one again. It was fucking terrible). While the FF films took a lot from the Ultimate version, and they were both terrible and poorly-received. All the stuff they tossed as silly and old is in fact what people LIKE.
I remember that bizarre period where writers and editors who knew better, like Millar and Ellis, were propounding this "widescreen" idea that the comics should be more like a Michael Bay movie. Was this UK condescension toward the presumed bad taste of Americans? Blatant opportunism? Whatever it was, it degraded the medium. MICHAEL BAY? The same Michael Bay who does nothing but shitty fucking movies? This is a model to follow?
But it did lend itself to unsatisfying comics that took 5 minutes to "read" basically pumped full of air that they could quickly release trades of and swamp the shelves, as Marvel's strategy has been(crowd the competition off via reprints) ever since the restrictions were lifted on how many comics they could put out. It ALWAYS WORKS, you see.
I'll just add: most particularly I hated Ultimate FF, and its Reed. Have you noticed just how many plots revolve around terrible consequences when that Reed does just about anything scientific? How plausible is it that he's allowed anywhere near the equipment? SUE is a more responsible scientist(and possibly smarter) than him.