Because HTML5 is not actually a subset of XML. Every element in XML must be closed, but in HTML5 certain tags (like <hr>, <img>, or <br>) are defined as "void" which means they have no nested content and therefore it's technically improper to close them (although I'd be shocked if any major browsers actually care about that). In other words, an <img> tag does not "open" the definition of a new section of the document the way <div> or <span> do, and it makes no sense to "close" something that was never "open" to begin with.

Perhaps they made it implicitly self-closing.

Haha I've known for years and I still use <br />. Usually the linter will nag me, but otherwise I just don't care enough to fix it.