Because realistically, that's more work than I'm willing to put into it. I wouldn't maintain that long-term. Especially because then I'd have to also sit through ads (99% of my YouTube use is from my TV via my PS5, so adblock isn't an option there), which would turn me off from using the platform, at all.
Premium is what works with the compromises I'm personally willing to make. And, this may come as a shock, but I don't want Google to get nothing, either. They need to be able to maintain their platform, which I get hours upon hours of use of every single day. I don't take issue with them making money in order to keep the lights on.