Unless Fox and Sony really just don't want to sell the rights back. Which is the case. They only have to keep making movies out of them, when they don't the rights revert to Marvel. But they do. (at least for one of them, I'm not sure if it was the case for both)
Yeah, that's what happened (or is going to happen? I don't know if it's a done deal) with Daredevil because Fox didn't want to give up the rights to the cosmic characters it's got as part of its Fantastic Four deal. All they have to do is keep rebooting it ever 10-15 years to keep if profitable.
Oh, why must I tell 'em again?
What must I tell 'em, my friend?
That we are born into sin
And we are worshiping murderers
Nothing but evil duplicitous men
I know you heard of 'em all
We should just murder 'em all
We should just murder and ball
We dance on the ruins we looting from all of they palace's walls