Photopea could just as well be monetized by selling licenses or by non-intrusive unpersonalized ads. When you have a great product making money from it isn't hard.

I think that 90% of ad tech is useless shit technology that just exists to take a cut of the money, without actually providing much of a value to anyone.

It's like realtors or financial advisors or other middlemen like that. The main reason for them being there is that it's a very profitable business to get a cut of somebody else's transactions.