Because Sony and Disney couldn’t come to an agreement on the future of Spider-Man movies his time in the MCU is done.
The whole dispute was because of money. Disney wanted a 50/50 financing arrangement and Sony wanted to keep the deal as it was with Marvel getting around 5% of the gross.
Sony has other films featuring the wall-crawler and other characters in the wings like “Venom 2,” “Into the Spider-Verse 2,” “Morbius,” and more in waiting.
Guess they figure they can get the same mass appeal that Marvel does by doing it on their own.