Or, and hear me out, maybe it was not a very good movie.
I get that critics have a different perspective because they'll see so many more movies and have a different interests than fans, but we've had decades now of them giving a pretty fair shake to superhero and popcorn movies and reviewing them on their own terms. If your sequel doesn't do anything particularly interesting in that space, then the property or the cast had better be so beloved that people will go see it regardless. I haven't really heard that anybody HATED Shazam 2, just that nobody gave a shit, and most of the review snippets on RT reflect that.
I watched it the other day on HBO. It was pretty trope-y and predictable. For me it just solidified the end of the Superhero genre because I don't feel like there's anything new they can bring to the table. It wasn't really a bad movie, it was just very flat and forgettable.
Falcon and Winter Soldier nearly got good when dealing with the un-snap consequences, finally a new idea, but then it failed to do anything interesting with it
I felt the same about WandaVision. They gave up the mystery about halfway through and it just became kind of boring and predictable after that. I'd say the same about Moon Knight too.
Like with horror movies, most releases will follow the same tropes and a few gems that do something new and interesting will pop up. Logan stood out in the middle of the superhero boom as new and interesting.
The superhero genre is currently coasting, but it hasn't ended and eventually something interesting will pop up.