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.
Too deep in yearning we need
something to hold, and something to keep
Innocent eyes could not realize
the meaning of "destiny"
But someday I promise that
you’ll start to notice what’s there on your shoulders someday -
Wings that are strong,
they will guide you along to the future that holds your fate!
Let me tell you a cruel angel’s thesis -
Flying high from the window, finding egress
There is something betraying all my memories,
burning pathos, emotion overwhelming!