Classes turn out to conflate two or three different, unrelated concepts. Better languages split those things out so that you can use them separately. (Believe it or not, Java actually advanced best practices in this area: in C++ interfaces were just a "pattern", and many C++ fans were sceptical of the value of making a language-level distinction between interfaces and classes).

When we say "A extends B" we mean a) an instance of A composes in an instance of B b) A implements the interface that B implicitly expresses c) this implementation, by default, delegates to the instance of A from a). Sometimes you really do want to do all three of those things. But often you want to do some subset of them instead. So the language should give you ways to do parts of that without doing all of that, and then "extends" should be at most lightweight syntax sugar over that.