Monero mining in the browser has been done by criminals without the consent of both hosts and visitors of hacked websites. The method of earning money via ads in the internet has severe downsides for both hosts and visitors: hosts can only earn significant money if they know their audience well, and if their visitors do not use ad blockers. Visitors have to fear data breaches of very personal data. Hosts have to make extra effort concerning data security and evaluation.
Why aren't many more websites using monero mining as a tool to earn money (legitimately by announcing it)? I'm thinking about newspapers, video streaming like youtube, social networks, even stackexchange etc. I would be perfectly willing to give computing power and the energy cost in exchange for those services. Are there any downsides of mining vs ads except for the energy cost the visitor has to pay?
Bonus imaginary internet points for answers that also provide information about the expected gain per visitor per minute for both monero mining and using ads.