The issue of Marvel movie rights is a very complicated thing. The reason it is so complicated is because in the 90s Marvel was worried about going bankrupt. In order for them to make some money that had to sell of characters to different studios like Fox, Sony, and Universal.
They sold Spider-Man and related characters to Sony and the X-Men and Fantastic Four related characters to Fox. They also gave character like Daredevil and Blade to Fox. However as part of these contracts if the studios doesn’t make a film in a certain amount of time then the rights revert back to Marvel. This is what happened with Daredevil, Blade, and a few other characters. This is why Marvel has been able to make the Daredevil Netflix series recently.
Spider-man has been able to be in the MCU because of a deal between Marvel and Sony that gives Marvel creative control of the Spider-man movies, but Sony gets the profits from the Spider-man movies. This has also enabled Spider-man to appear in movies like Captain America: Civil War and the upcoming Avengers: Infinity War movie.
There are rumors that the Fantastic Four will be introduced into the MCU as part of phase four, since Marvel has announced several untitled films for 2020. I hope that the Fantastic Four are able to come back to Marvel soon, since the last few Fantastic Four movies produced by Fox have been so bad. I also hope that one day the X-Men will be able to be in the MCU so that we can see them interact with characters like Iron Man and Captain America.