It's not that they're ideological hard-liners, but looking at the relationship between 20th Century Fox and Marvel Entertainment, it's hard to see a deal happening. Even when it looked like there was zero chance of the Spider-Man film rights being handled by Marvel Studios, Sony Pictures and Marvel were at least on good working terms with one another and cooperated on the Spider-Man IP to their benefit like Marvel giving up their cut in the box office in return for merchandising and television rights.
20th Century Fox and Marvel on the other hand don't seem to be getting along at all from being unable to make rights swaps regarding Daredevil/Silver Surfer, Marvel rather pettily cutting off Fantastic Four merchandise, 20th Century Fox making a bad Fantastic Four film for the sole purpose to keep the film rights, etc. And you can't blame one side over the other on this one, these two entities just do not seem to have a good working relationship at all. The purchase of Lucasfilm by the Walt Disney Company probably didn't help things either.
In order for any deal to get made, a good working relationship needs to be there to begin with. The Sony/Marvel Spider-Man deal happened because even though Marvel had no shot at getting the film rights, they still had a good working relationship with one another and it allowed Sony and Marvel to work a deal that was mutually beneficial to the both of them. The Fox/Marvel/Disney relationship on the other hand is just bad and they need a bit more than just zeroes to get it going.