Why aren't the fantastic four in avengers?

Answer 1

They have their own team in their own comics and movies.

As to why they don't appear in the Avengers films:
Until recently, each Marvel superhero was treated as the only superhero in his/her world, except in crossovers and especially in the movies. It would be logistically difficult, not to mention prohibitively expensive, to assemble the entire Marvel pantheon in a single production. However, this would not preclude "cameo" appearances as occurred leading up to the breakthrough Avengers movie. In the comics, this was often provided as an aside panel to explain why one hero or group has to save the world "by themselves."

Answer 2

They say there is going to be an Avengers 2 and that the Black Panther is going to be in it