Well, let's just look at the quality of Sony SH movies with and without Marvel's involvement. That's fair. I liked SM 1 and 2 and it doesn't matter whether or not CBMs were seen as a long term business venture, but those were 20 years ago and it doesn't really speak to what's going on today. If you hire iffy execs, it's going to have a negative effect. As
@Iceman stated, after SM2, there was a sharp drop off in quality and TASM2 was one of the worst CBMs I can think of. It wasn't until Marvel got involved with Holland's current SM that the quality went up. Spider-Verse was considered good. That's fine.
Now........let's look at that non-SM characters that Sony made about without Marvel's involvement. Venom made money and that's ALL you can say about it. Carnage was a (edit: pretty popular) disaster and so was Morbius (only not popular). Are we starting to see a pattern? I agree that it's too simple to say "Sony bad, Marvel good" because even a broken clock is right twice a day (unless it's a digital clock, but you get my point). By and large, Marvel has made pretty good CBM movies if you look at the overall opinion and Sony just has not. Sony has "proved" nothing when it comes to making decent CBMs without Marvel's help. Had Venom and Morbius been well received, that would have gone a long way towards offering "proof", but, to any unbiased observer, that just wasn't the case. Marvel, on the other hand, HAS shown they can make movies that are consistently well received has developed a detailed universe that includes, now, a multiverse. Those are pretty much the facts.
Add to it the fact that Sony just doesn't have depth when it comes to new characters. Compare characters like Kraven, Morbius, etc. to characters like Dr. Strange, Hulk, Thor, CA, IM, BW, FF, X-Men, etc. and it is easy to see exactly why Sony will be very, very challenged to create a universe that will be successful in an ongoing way. Even without the recently purchased IPs from Fox, Marvel had much more depth of characters. Add to it the executives at each studio and that only adds to my take on the 2 studios. Sony, frankly, has a LOT to prove. Marvel does not. Does that mean Marvel will not start making a bunch of stinkers? No, but not many people are putting money on it.