> 99% of the time those sites could be plain HTML and CSS

Some of that css and html doesn't run reliably and consistently on all browsers. A visit to sites such as canIuse helps build up an idea of the extent of the problem.

Also, you seem to ignore the fact that JavaScript frameworks use said html and CSS extensively, and provide the necessary abstractions to bridge the unreliability gap in addition to introducing features that html and css do not support.

What I really find funny about this issue is the fact that this type of claim implies that virtually all software engineers who for some reason aren't html+css purists are utterly incompetent and completely unable to assess any technical tradeoffs. Imagine yourself walking around with the belief that an entire field is manned by people who don't have a clue about what they are doing.