While that is true, Infinity War was an exception.
Infinity War did not just focus on what Disney's Marvel focuses on ( comedy and action ), it also had a lot of characters growing together.
Infinity War is underappreciated in my opinion, that movie was a masterpiece.
Also I never saw Spiderman 2 or 3 in the Sam Raimi trilogy, but I seen the first one recently. The Green Goblin was pretty interesting but he did not really build me up enough.
I haven't seen Infinity War but the reason I haven't seen it is also probably the movie's biggest problem - it relies on me watching the majority of the MCU movies to really feel an impact.
I usually don't mind this. Things like Harry Potter and Kingdom Hearts are the same way. But the difference there is that they use a more limited set of characters who are built up over the course of the series. Harry Potter is the main character of all the Harry Potter movies for example, so him and his circle of friends see a constant and consistent rate of development and growth throughout the series. That makes it much easier to get attached to the characters in that case. There's also less movies to watch or books to read.
But with MCU, I have to watch like, 20 films, starring different characters who get varying amounts of screentime and development across this gigantic spread of films. And if I get attached to a character with a relatively small amount of screentime, and don't care much about the rest, then Infinity War isn't gonna have much of an impact on me.
My favourite Marvel super hero is Spider-Man and I already know he has an appearance in Civil War, his own Homecoming movie (where the character was fun but not exactly compelling), and then the big finale in Infinity War. Compare to a character like Captain America who has three movies with his name in the title and a starring role in the Avengers movies and cameos in other movies (like Homecoming) and already there is this huge disparity in how relevant certain characters are (and while I have seen some of the films starring Captain America, they haven't made me fall in love with the character).
And since the MCU films are made for people with an established interest in Marvel properties, I essentially have to be a Marvel super fan to truly get an emotional attachment to many of these characters. And Infinity War, from what I have heard, completely relies on that, and is a mediocre and confusing film at best without the pre-existing franchise love and knowledge.
Also straight up, Infinity War is far from underappreciated. It's one of the most successful films of all time, objectively.