It's entirely possible, I agree, but as of right now, there's only really two ways to show a person ads - targeted or not targeted. And that applies to all forms of advertising really, not just online. I don't know enough about marketing to say how long it might take to develop zero knowledge ads but in terms of funding development and hosting we're very long way away from that being a possibility.

And the ethos of the Fediverse was, in part, formed around the idea of not being served ads that were either utterly irrelevant and thus irritating or scarily relevant and thus unsettling. The Fediverse is pretty closely aligned with open source and privacy philosophy. I think any instance that runs ads is going to see either no revenue as they'll be adblocked or the instance will see a sharp drop off in users.