They don't need built up they just need personalities. There's a reason why mystique Xavier and Magneto and wolverine are front and centre and it's because not everyone wants to watch 5 care bears.
The Avengers have bickering. The xmen have that with the Astonishing team. Not everything has to be an end of world scenario like the dps. The astonishing team have the tone to do both. They're versatile.
Gotg was set up in one movie
While the fox X-Men try to hard to be taken seriously the Avengers movies have self awareness and know that they just need to make something entertaining while staying true to the source material. And that approach seems more acceptable. Same for deadpool it's the opposite of every fox movie