Spider-Man is no longer lucrative under Sony. Fans want to see Spider-Man in the MCU. It's this simple. Work out a co-production deal with Marvel Studios. Why not? Sony and MGM have a co-production deal for the James Bond franchise. It's not a huge stretch.
The Amazing Spider-Man reboot franchise while not an outright failure has been a major disappointment. Sony is going to suddenly drop even more money, probably well over half a billion dollars, into spin-offs of ASM2 when audiences were so lukewarm on that film? It makes no sense.
The best option is to bring Spider-Man into the MCU which would breathe new life into the franchise and would get fans excited about the character again.
A Spider-Man Cinematic Universe is never going to work. That is not what moviegoers want.