Because it’s never actually useful.

You can’t use that information to know how much actual space it takes (in storage) as nobody sane stores UTF-32, you can’t use it to know much much logical space it takes (aka the user’s interpretation), you can’t use it to know how much visual space it takes (not that you can ever get that), and you can’t use it to segment or process the text.

A length in codepoints gives you nothing that’s really actionable, at least not that you’d need outside of a context where you could easily obtain it otherwise.