Same reason why datasheets are still PDFs. It's a reliable, long lasting and portable format. And while it's kind of ridiculous that we are basically emulating paper, no other format fills that niche.
It's the niche HTML should be able to fill, since that was its original purpose, but isn't, since all focus over the last 20 or so years has been on everything else, but making HTML a better format for information exchange.
Trivial things like bundling up a complex HTML document into a single file don't have standard solutions. Cookies stop working when you are dealing with file:// URLs and a lot of other really basic stuff just doesn't work or doesn't exist. Instead you get offshot formats like ePUB that are mostly HTML, but not actually supported by most browser.
I really wish there was a standard way to package html with JavaScript for offline viewing, basically to treat it like a pdf but with fancy things like interactive controls.
It's the niche HTML should be able to fill, since that was its original purpose, but isn't, since all focus over the last 20 or so years has been on everything else, but making HTML a better format for information exchange.
Trivial things like bundling up a complex HTML document into a single file don't have standard solutions. Cookies stop working when you are dealing with file:// URLs and a lot of other really basic stuff just doesn't work or doesn't exist. Instead you get offshot formats like ePUB that are mostly HTML, but not actually supported by most browser.