I'd say it was politicized, poor, hackneyed, boring writing more than anything.
If something makes me laugh, then I laugh. If something is thrilling, then I am excited. If something makes me feel, then it is relatable. But that all requires good writing.
The images on the screen are incredible, which is part of the problem. The CGI makes things unbelievable as they strain credulity and remove the suspension of disbelief.
Multiple stories told about the same "hero" isn't a bad thing, as long as character development and plot are engaging and delightful. Unfortunately, the writing has become increasingly preachy and tedious. It seems the writers and producers are trying to push their own personal political agenda, rather than explore more universal themes. So perhaps the sequels, reboots and MCU aren't the problem, as much as the writing is poor in the sequels, reboots, and MCU.
I will say though with respect to the MCU, that after Endgame, where do you go? The stakes were so high, how do you ramp them up any higher?