I'd say that to begin with Lois Lane's only purpose was to be a victim
so that Superman had someone to go into action to rescue all the time,
but over the course the last 4 decades has become a character in her own
right. MJ, in the Spider-Man books, has had the best of all worlds: she
has actually almost had the evolution of a real person, having started
off as a friend of Parker's from school who dated more than one of his
friends (and a frenemy) before becoming Parker's girlfriend and eventual
wife. Meanwhile, Parker also went through a series of other girlfriends
and didn't spend the early days of the title pining in unrequited love
for the redhead who would eventually become "the one". That is where the
Silver Age Marvel really turned the tide in treating it's characters
like real people instead of cardboard cutouts.
But to be fair to the MCU, the comics have had 12 issues/year for
decades to play out those story arcs, while the movies only have up to
about 2.5 hours at a pop to introduce a villain, set up a plot, have
some action sequences, and resolve the story, and if part of that time
is also the title character's origin story then you run out of room to
develop peripheral characters pretty quickly. If these were TV series
like "Daredevil" they could breathe a little more life into the people
who fall into the titular superheroes' orbits, but the feature film
format doesn't offer much latitude for that.