Honestly, I think this is the biggest factor.
Not every single Infinity Saga movie was perfect, but it was all building upto something big that was unprecedented in cinema. There was a sheer momentum to the enterprise that carried everything along in its wake - audience interest, media interest, mediocre or average entries...everything. That momentum disappeared after Endgame.
Now I'm not saying that the MCU necessarily needed something big to build upto in order to succeed...but not having something puts a greater burden on each individual entry to excel on its own merits. And frankly, that hasn't really happened over the past half-decade.
The first 3 phases each had some truly standout movies that helped sustain long-term investment in the brand and kept up the aforementioned momentum. Phase 1 had Iron Man and The Avengers. Phase 2 had Captain America: The Winter Soldier and Guardians of the Galaxy. Phase 3 had Captain America: Civil War, Black Panther, and of course Infinity War/Endgame.
How many such movies have we had post-Endgame? Spider-Man: No Way Home for sure, but that's arguably more a Sony movie, and a celebration of Sony's Spider-Man legacy. Wakanda Forever, maybe? Which somehow didn't exactly thrill the world as much as the original did, and mostly functioned as a heartwarming tribute to Chadwick Boseman. And that's about it. I personally enjoyed a lot of the other films, but they didn't set the world on fire.
And sure, there are a lot of other contribuing factors. COVID, superhero fatigue, the loss of Chadwick Boseman (who was supposed to be part of the new MCU 'Trinity' alongside Brie Larrson and Benedict Cumberbatch), and now the Jonathan Majors situation. But honestly, the quality of the overall output has gone down. I dunno, maybe with the TV shows they're all spreading themselves way too thin?