I'm happy to disclaim that I have nothing to do with the mess that is the HTML standard! But if I had to guess I would say that the ending tags in HTML add a modicum of redundancy that allows for some of the crazy/creepy/smart error correction browsers have been doing for decades:

You can close your P tags or not; if you don't, the next opeining one is a new paragraph, with the previous one assumed closed. Similarly with LI tags in lists or the multiple levels of stuff in TABLE.

That makes HTML "code" marginally easier (at least in terms of effort) for humans to write and maybe also read, and for programs to error-check and correct. It's also led to waves of shitty HTML code and generations of smarter programs to guess the intentions of stupid HTML generators.

XML, being more strictly defined, could have followed the lead of Lisp and done without the closing tags. I guess people felt the need for a security blanket of closing-checkable tags, or its slightly better human-readability.