Users might prefer, say, that 99% of the time a data selection widget behaves the same whenever accessed by their browser. When devs choose a JS UI library then that's usually what users get. Wouldn't it be good to have all tables have the same features (sort, collapse, column-select, data export), rather than only having rich features if the dev implemented them, duplicating the effort of millions of other devs.

There's a sliding scale here, with html at one end and each page ships a parser and libraries at the other - why have an img tag, just let the page developer implement a JS based interpreter?