Primarily because I already pay for YouTube Premium on my main devices, but some of my other devices aren't signed into that. I'm not paying for it twice, especially not on my work account, that would be inappropriate. I already support most creators I'm subscribed to via Patreon; the ones I don't tend to not accept payment at all, usually smaller channels that do it just for fun. These channels also aren't
big enough to be a YT partner, so they aren't getting any ad revenue; there's nothing to "rob" as you put it.
Secondarily, because I already run an ad blocker for the rest of the web, and I'm not ever going to turn it off on principle. I view ads as a malicious tax on my attention and a threat to my computer's security. They simply don't get to run on my machines. Monetize your content some other way, I'm happy to pay for it once you've earned my respect. (You do that by making good content.)
(This is also why I stopped watching regular TV; too many damn ads.)
Notably, I don't really "browse" YouTube. I have a small fixed set of subscriptions whose videos I watch. I use my content blocker to remove the "recommendations" sidebar and the end-of-video grid, because I find them distracting. So, yes, I run my "ad"-blocker even on my YT Premium devices because I gain other UX benefits from doing so.