Because when function components first came along that's what the whole of React was supposed to be: "functional" UI, using things like redux or mobx for state. Plenty of guides still use that terminology even though React's homepage now uses "declarative".

That said I think they were always misusing the term, I remember this because I used to complain to people that react itself was declarative, not functional, and just gave the option of writing in a functional style.