A few reasons:

• The userbase on the Fediverse is not big enough to support a donation-based economy.
• The userbase on the Fediverse is not big enough to support an ad-based economy. Even if by some magical powers we got an ethical ad network working here (which didn’t track users and focused solely on paying people by the opportunity of broadcasting their inventory) there wouldn’t be enough eyeballs to attract advertisers.
• The userbase is still anti-business.
• For all its faults, Youtube is hands-down is the platform that pay the most to content creators.
• Content creators are not willing to spend their time building out audiences on new platforms. Principles be damned, they will just go where the money is.

I’ve added support for crowdfunding to Communick earlier this year, and even people who are active on the Fediverse and have a vested interest in having monetization alternatives turned it down. This is why all we see are these completely fringe ideas that can only appeal for the get-rich-quick crowd.