If we make case classes final by default, we need a way to make them open (and no, abstract is not good enough). Overall, things get more complicated. open as a modifier would require we allow identifiers to be modifiers (since there is no way we can reserve it as a keyword now). That would make things even more complicated.
Since we now do have a way to make lightweight final case classes using enums I believe there’s less need for this. If you want lightweight syntax you are well advised to use enums anyway.
In retrospect I also would have liked final to be the default (for all classes), but it’s both too late and not important enough to reverse this now, I feel.