Because despite the common request to "let me pay a monthly fee to not see ads", the vast majority of people are too cheap to pay up. This is why ads are as prolific as they are.
For me it's a bit of a Catch-22. I don't watch much YouTube content anyways, but I've reduced my consumption over the years because of the ads. From what I can tell, there's nothing I want to watch anymore because I no longer know what's there (and of value to me). So the prospect of paying is absurd for me as I'd be buying something with no real value except to remove an annoyance on the few occasions each month I bother to watch something on YouTube. If I had a clearer reason to increase my YouTube consumption, I'd entertain paying.
Other video services have clearer value to me in the content they provide so I pay for them. I suspect I'm not alone in this.