The Marvel Cinematic Universe allows for some of our favourite Marvel Comics characters to be brought to life and story arcs to be realised on the big screen. Part of the excitement is seeing the on-screen translation of fan favourite interactions, iconic panels, and pivotal conflicts play out. One example being Spider-Man, trapped under rubble, struggling to get free in Spider-Man: Homecoming being wonderfully reminiscent of the cover of Amazing Spider-Man issue 33.
Lots of characters from Marvel Comics have appeared in the MCU so far and, for the most part, have interacted in ways similar to how they would in the source material. With the intertwined nature of the films, it seems to be only a matter of time before one hero meets another.
This is not always the case, though. There are times where characters are unavailable or simply replaced by others in the MCU. Any number of reasons could be the cause of these replacements, from a lack of rights to the characters involved, to a change in creative direction. Change is often a good thing, but sometimes in the MCU it's just simply necessary