I agree. Sony was in a bind with Spider-Man. Their entire movie division was in a bind. They had a huge financial incentive to make a deal with Marvel and Disney. They needed money, resources, and legal flexibility. It made sense for them to work with someone else. That's what companies do when they're desperate. I think that's part of why Marvel/Disney is more willing to work with them. They know they have more leverage.
With Fox, I don't think that leverage exists. Fox is doing well for itself. Other than F4, it has turned plenty of profit from their X-men movies. Adding TV shows to the mix only sweetens the pot for them. They have no real incentive to work with Marvel/Disney. They also have more leverage, I think. Look at movies such as Logan and Deadpool. Those movies cost a fraction of what Marvel/Disney spent to make their big MCU movies and they still turned plenty of profit. There's just now way Marvel Studios ever would've made a movie like Deadpool. What they're doing is just so incompatible with the MCU at the moment that it's hard to imagine any deal ever happening.
I think that total discrepancy is what drives the divide between the two companies. I think that divide has been bridged, somewhat. What ever animosity was going on behind the scenes has been tempered. We're seeing that with Marvel's willingness to work with Fox on TV shows. But they have a long way to go before they ever work together on a movie.
It'll be interesting to see what happens to the F4. Fox lost a lot of money on the last movie. At this point, holding onto the rights might just be too expensive and risky. If Marvel and Fox can make a deal with that, then maybe that sets the stage for more deals down the line.