Direct cost efficiency isn’t the only factor: I think site operators value the simplicity of not needing a relationship with each visitor (especially internationally) or needing to ask permission to access content, and visitors like that they don’t have a taxi meter ticking as they browse around.
I’m definitely not saying the system is perfect but any time something is this resistant to change I would assume it’s better at a factor we’re not considering. Ads certainly aren’t perfect and the major publishers would drop Google/Facebook in a heartbeat if they thought they could afford it, so I would look at proposed replacements from that perspective.
A blockchain based system has a few obvious challenges but the biggest one for me is the assumption that enough people don't like ads enough to pay a comparable amount of money & deal with thinking about transactions. That experiment has been tried enough times that I think the failures are an important lesson about revealed consumer preferences.