Although the MCU consists of tons of movies, the MCU story is basically the Avengers vs Thanos with the other movies basically being side-content/the appendices to fill in the backstory of what other characters were doing. Now that Thanos is dead and the OG Avengers disbanded, the story has effectively ended which is why everything in Phase 4 and 5 are so lacking imo.
I think it's a little more complicated than that.
I think Phase 4 and 5 are suffering from a few different factors.
1. Some fans are being resistant to the introduction of new characters. They miss the old gang and want 'em back.
2. Especially coming off of Phase 3/Endgame, Phase 4 felt unfocused and like it had no unifying concept to it. It was just a collection of one-off movies that didn't seem to be building towards anything specific. For better or worse, audiences have become trained to accept the main story of the film, but be constantly waiting for that end credit hint at what's coming next and what we're building towards. With Phase 1, it was the formation of the Avengers. Everyone knew it, everyone could see it, everyone knew it'd be big. With Phase 2, it was a much more mixed bag. The films ranged from AMAZING (e.g. The Winter Soldier, Guardians of the Galaxy 1) to decidedly "Meh." (Iron Man 3, Thor 2, and frankly I'd put Age of Ultron in there, too). Phase 2, as a phase unto itself, wasn't really building towards anything. Age of Ultron didn't feel like the natural progression of the developments of Phase 2. It felt more like "We have to do another Avengers movie, so here it is." BUT, we'd seen Thanos, so everyone who knew comics knew we were headed towards an Infinity Gauntlet confrontation of some kind. Phase 3 was the culmination of that, and it was epic. But Phase 4? What's it about? Nobody knows. The post-credit sequences seem unrelated, and just kinda there. And that's kind of how all of Phase 4 seems.
3. Phase 4 was also the first phase to really delve into expanding it via television. Phase 2 and 3 had some tangential connections to Agents of SHIELD and Agent Carter, but Agent Carter was only very distantly related, and Agents of SHIELD always felt subservient to the main cinematic approach. With Phase 4, you had big-budget productions furthering the story (but which, again, felt unconnected) introducing more characters, and doing so in a much different format. I think the shift in format (a) required people to have Disney+ to access the new material, and (b) was jarring to some audiences. It's not the bite-sized chunk that a movie is. The most significant of these, though, was Loki, which introduced Kang and the multiverse, and which (I thought) set the stage for what the big confrontation will probably be at the end of Phase 5.
4. With Phase 5, the trend of television and film both supporting the phase more directly continued, and I think continued to be confusing for people. BUT, we now at least kinda sorta have a direction. The thing is, the Kang storyline is harder for people to get a handle on than the Thanos one. Partially that's because Kang is an inherently more confusing "character." (Because he's more than just one character.) But partially it's because I think Kang as a threat is simply less well known. I haven't followed the comics closely, but I think the main storylines about Kang hit more in the 80s than the 90s. As a result, the target audience for these films is way less familiar with Kang as a threat, and with the storyline to kind of guess where things are headed, and so it all again feels a bit unfocused.
5. Then there's the whole thing from Phase 4 with the Eternals and hints at really ramping up the magical side of things for Marvel with Moon Knight, Wandavision, and Werewolf by Night. I don't think anyone has
any sense of where that's all headed. Unless the Celestials end up being the Big Bads for Phase 6, I don't see how the Eternals fits in
at all with any of the other films. Right now, it feels like a totally separate story that has been largely ignored, kinda like how the Inhumans TV show was basically memory-holed altogether. Likewise, the magical stuff just feels like "Oh, by the way, it's not just Doctor Strange. There's all kinds of magic in this universe" and...um....that's it. Just a friendly reminder, folks!
Overall, I've continued to enjoy all of the material Marvel has put out for these two phases, but I think they are
feeling less focused to audiences, and the spectacle of Marvel films just isn't a draw by itself the way it was 10 years ago. It's not about "bad stories." It's about "We have no idea where this is going, and frankly none of it is all that special anymore." Audiences are kinda spoiled now. Back in the day, superhero movies were rare things, and a big damn deal. Now? Meh, they're like busses. You miss one, another will be along shortly. And the two new phases haven't done enough to make themselves feel special by feeling like there's something bigger going on.