It's not that people care about quality, but that people expect things to "just work".

Regarding the point about accessibility, there are a ton of little details that must be explicitly written into the HTML that aren't necessarily the default behavior. Some common features of CSS and JS can break accessibility too.

None of this code would obvious to an LLM, or even human devs, but it's still what's expected. Without precisely written and effectively read-only boilerplate your webpage is gonna be trash and the specifics are a moving target and hotly debated. This back and forth is a human problem, not a code problem. That's why it's "hard".