But the biggest problem with the Spider-Man franchise atm is Sony's "creative control". Allowing Spider-Man to join the MCU in a co-production with Sony where Sony retained control risks both franchises. Sony could do something in the Spider-Man films that could cause damage to the whole MCU, and that is not something that Marvel is willing to risk. Marvel would need to have creative control, one of the most important things about having a large unified universe like the MCU is making sure that everything fits, letting Sony do whatever they want would not work.
Ultimately Marvel would like Spider-Man in the MCU but they don't need him. Lets face it, Marvel made far more from the merchandising than Sony made from the film itself.