I think a lot is because Hollywood became a Henry Ford production process, one part feeds into the next so they'd have empty studios and workers idle if the next idea isn't ready to go.

Also it literally doesn't matter, this marvel film has literally the same plot and jokes as the last one? That's ok we cooked up a drama where we pretend villainous gamers are against it to get people talking about it, we seeded stories into the media we own about it and forced our celebrities to pretend to love it...

They can make the absolute worst shit and as long as they link it to something vaguely related to some culturally significant thing it'll be huge, even more so if they can link it to a social divide or political division they have no intention of ever actually caring about.

Childhood toy + social flag = money, it works for comic books 'i had a the flash t-shirt when I was six I have to like these new films', it worked with Barbie 'this proconsumerism corporate tat which was heavily criticised by notable feminists has made a film attempting to shoehorn social progress back into a corporate friendly sales generating mush, they say the baddies don't like it so I have to go see it!', and it works with endless sequels 'this franchise now makes zero sense, has the most painfully predictable plots, has gone so far off the rails jumping sharks that literally nothing makes sense and there are zero stakes to any of it which totally ruins everything that made the first one good...' and you can't even tell what I'm talking about with that because it's everything (i was thinking john wick btw)

Make something actually good and no one will care unless the media circus tells them to, that's how you get s flop. Make something even slightly changing intellectually or from a certain point of view and instantly most your audience is gone or angry, but be like Barbie and put sparkle on social concepts 90% of the world has agreed on for decades while actively avoiding anything more contentious then you don't need to worry about alienating the audience or going over their heads.

And for some reason people just won't stop watching it, they won't watch indy stuff made with passion or small budget things no matter how good they are because they HAVE to see the big releases, like you'll lose touch with society and be unable to make friends if you don't force yourself to endure at least a dozen painfully dull industry movies a year.