Take Citizen Kane. I'd call it a 7/10 film and kinda boring, but that's because I'm seeing it in a "post-Kane" world. At the time, however, it was so revolutionary that people still call it the best film ever made. Nowadays it's quite standard because everyone has been building on it ever since
While I'm not going to disagree with you regarding some art, there is a point to be made that what to us, un-cultured people, may seem like random scribbles could actually be something revolutionary. We simply don't notice it because we are missing the context.
I do believe, however, that you are right and that there's an element of luck too. But that's not exclusive to art - how many technologies are we using right now not because they are the best but because they got a lucky break at the right time? And then again, while many good painters may never get their big break, I doubt there are that many bad painters who get famous.