Re: wat if they built up to x men, like avengers?
Because if there's a 1st X-Men movie, people would be looking for a couple of characters. Not an origin story just for 1 X-Men character. Whether its the cartoons, videogames or a film series, they would always start it with the X-Men already assembled. Then the flashbacks/origin stories come in later.
X-Men already assembled = more appealing to the mainstream viewers
Origin story for 1 single character = not so much especially if its not Wolverine