I was thinking about it and I think part of the problem is that she lacks a torch passing.
Like, as much as people bitch about it, and as much as some of that bitching is deserved, Spider-man getting a torch pass from Iron-man kind of cements him into the MCU, similar to War Machine and Rescue Pepper
if they wanted to go that route.
Cap has his proteges in Sam and Bucky.
Hawkeye of all people gets to mentor Wanda.
A lot of the newer Avengers are tied in some way to the old ones, but Carol, I guess because they had to make her strong independent woman, has no ties to anyone. She's this rogue element with the least amount of connection to the rest of the MCU.
Which is interesting because even in the comics, she at least had that. You could say "Well, she's a better fit than Monica because at least she had a connection to Mar-vell" and you could kinda see the legacy there. She had a nonsensical tie to the X-men. It wasn't much but it was something.