I think the parent has a good point: browsers don't do anything with these tags for sighted users, who are unfortunately the majority of developers. If they were to notice benefits to using semantic tags, maybe they'd use them more often.

I use reader mode by default in Safari because it’s essentially the ultimate ad blocker: it throws the whole site away and just shows me the article I want to read.

But this is in opposition to what the website owners want, which is to thwart reader mode and stuff as many ads in my way as they can.

It’s possible good accessibility is antithetical to the ad-driven web. It’s no wonder sites don’t bother with it.