Because ad revenue is tightly-clustered. It's not ad revenue from millions of different accounts; its one big ad agency and it can terrorize your company by pulling all ads at once.
Discussion
Which could be avoided by not relying on ads. But that clearly isn't possible from the numbers I've seen. I don't see Nostr changing that as a technical solution. It'll require a shift in culture and user expectations. It's already technically possible on Nostr as far as I can tell. And yet the masses aren't pouring in. There's more to it than technology.
Well, we don't have any critical mass, yet. 10% subscribers is more than enough to carry a freemium video model, but 10% of what number?
If you think of what our gitserver costs, you'd probably need a few dozen high-paying subscribers, to get a video hosting service up and running, which means you need thousands of regular users. Until then, someone needs to just bleed the cash.