I can already hear a few voices calling (or writing), "that's because your bait sucks
Look, it's not like "we" who submit books are not aware of the limitations of HTML. BUT, we just take a look at the TEI files and say "Good lord, I am not going to do that!" [because TEI is targeted at Grad Student Academics] and then we look at RST and we say "Good lord yet another set of 1970s little troff-like escape codes to memorize!" And then the HTML generated is a mess, and the rendering on the end user's device, while not horrible, is not great either, so what has been accomplished at the end? It is one thing to say "well, the rendering could be improved" but if you wanted to make the format(s) attractive to anyone the *least* one would need to do would be to create competitively attractive results. It is hard for these things to compete against HTML because HTML is, well, like everywhere. Unfortunately HTML has some really nasty bits when it comes to writing books. Among other things people "use the wrong tags" because the "right" tags are simply not there.