Sponsors pay more upfront. If creators are only using sponsors than their whole back catalogue is basically valueless. If it costs a creator 2-10 cents a month to host a video (based off S3 pricing), but they only made 1000$ on it upfront when the video was made, overtime the back catalogue becomes a pretty significant financial burden if it's not being monetized

Also it's worth keeping in mind that many people are also using tools to autoskip sponsor spots, and the only leverage creators have for being paid by sponsors are viewership numbers.

Patreon is irrelevant, that's just like Nebula, floatplane etc, it's essentially a subscription based alternative to YouTube.

Discoverability is pointless if the people discovering you aren't going to financial contribute. It's the age old "why don't you work for me for free, the exposure I provide will make it worth your time", that hasn't been true before and likely isn't here. Creators aren't looking to work for free (at least not the ones creating the high quality content we're used to today)