See the spec document for the rationale. In short: The browser already has a good and safe parser, and knows best how it will treat which elements. An external parser written in JS is overhead while likely to be worse than/outdated compared to the browser. (In general security practices, parsers being used for validation being different than the parser used for processing later is somewhat of a smell, because it makes it more likely you can sneak something bad through a special case)
If it’s part of browser, then it’s likely up-to-date with the latest features and quirks of the browser.
Is "PHP had something like this once" your only argument?
Given I've seen some CMS's double escape html character entities and other such bad uses of sanitation filters, I think it is a reasonable concern to think about. Forcing people to source a library would make it clear what their intent is in terms of cleaning things up.
Considering the history of mysql_escape_string it's an important argument