@jaffathecake because, while being much maligned in the last decade, XML got something right, and it's this part of the spec. HTML's self-closing behavior is obscure and confusing and hazardous.
Hixie should have fixed this in HTML5 to allow authors to opt in to the sane thing without going full-on XHTML (not that there's anything wrong with that; cf <https://www.nayuki.io/page/practical-guide-to-xhtml>).