That's the point. Someone finds something not successful, sees that they could make a lot of money off it, takes it for their own, has copyright over their new content. The labels you're talking about would gladly just mine what was released ${term}+1 years ago.
The point is that everything is copyrighted by default, unless you release it into the public domain.
> Right now if I want to sample a song made 50 years ago I still have to pay thousands to license it even if everyone involved has long since left.
And that's right. Why should someone else get to make money off someone else's work without compensating them for it, if they haven't released it to the public domain?
You're assuming that copyright protects the big labels -- it protects your creations too. The most famous case of this is Charles Dickens: https://foleyhoag.com/news-and-insights/blogs/making-your-ma...