[csallen]

Honest answer to your question: Because they have to-do lists a million miles long, and "make an HTML-only version" is often at the very bottom of that list, because it may require high investment while providing few benefits relative to other things on the to-do list. Also, because "giving users control" can mean a million different things… colors, fonts, sizes, dimensions, HTML vs JS, etc.

[chrismorgan]

And if it’s not the same code base, it’ll probably be buggy, because it’s not the UI people normally use.

Server-side rendering of the same UI avoids that hazard, at a much lower cost.