We remember the good old stuff because the bad old stuff wasn’t worth remembering. But we see the good and bad new stuff both as it’s happening.
So old stuff is automatically better but only because the bad stuff got throwed away already.
I think you hit the nail on the head. It’s exactly the same reason why old music seems so much better. The old movies you watch are just the best stuff at the time it was made. Sure, the old days got Star Wars, but also got crap like Slapstick of Another Kind and forgettable experiments like Modern Problems.
Now, I prefer the aesthetic of older movies (mainly pre-1985), but that’s just a preference and doesn’t say anything objective.
Another reason could be the shift in style of films. Everything changes, and so does film, and so it’s also likely your inner misoneist judging new movies.
Also, the MCU stands for Milked Cow Udder.