To everyone, different styling works best, browsers doesn't provide enough option to configure the looks of those tags without having to break other application websites. Also, people often finds popular browser's default look unreadable without CSS. Since most sites comes with heavy styling, users never configure their font, font-size etc defaults, browsers also expect everyone to make heavy use of css or JavaScript, so they don't seem to bother making default decent. For all that, I think it makes more sense to have something different than html.
>> they don't seem to bother making default decent
Could you share your definition of "decent"?
How about calling up D. J. Bernstein's website (https://cr.yp.to/) and have a look and see if you can read it OK. Despite my ageing Mk I eyeballs, it looks just fine from here. It's also almost completely bloat-free and so loads and renders in a fraction of a second.
Medium and many other sites can be almost as good as this once you block all scripts.
