It routinely doesn't, but Tolkien does. And what examples are you thinking of in post-apoc? Because the original Mad Max films centered a single hero, and even Fury Road has 2.5 PCs. Generally I don't think multi-PC parties work really well on film. Even LOTR had to cull the story to just 2 PCs, Frodo and Aragorn, reducing Legolas, Gimli, Merry, and Pippin to shallow action support or comic relief. The ensemble story works better in prestige TV across genres.Even fantasy routinely doesn't do parties though, and small survivor bands in post apocalyptic settings work just fine for 3-7 characters (to pick just one example).
It certainly exists, but Superman, Batman, and Spiderman are all way more popular than X-Men. Even the MCU is full of films that focus on a single character.It's also not like the 3-7 person superhero team isn't common, either.