Apparently Disney was asking for a ridiculous cut of the profit to continue universe sharing, like a full 50%. I don't blame Sony for refusing that deal.
Thei current Spider universe is already set, with Tom Holland apparently willing to play the part for basically forever, so the next one at least will be fine, (in terms of box office if not quality). Venom somehow did well despite being garbage, and Spiderverse is one of the best comic movies ever madeand won an Oscar. Oh, and Sony just bought Insomniac games who made that awesome Spiderman game.
The Sony handling Spiderman right now isn't teh same Sony that was trying to rush the SInister Six and made Amazing Spiderman 2.
Outside of all the Iron Man stuff, there's not a whole lot Spidey needs to interact with, especially since all the other avengers were older than him, and Tony was the only one he really got time with.
It's also possible they can still make deals to have Spidey show up in Avengers, just not have their hands in the solo movie production.
Of course, maybe they'll change their minds when Morbius, Kraven, and Silver Sable all flop.