The simple answer is they have low market value and aren’t a good investment for anyone - the student or the nation. Yes we need some researchers but not as many as we train.
The value is that obtaining a PhD involves deep research into something beyond what anyone else has done. Even if it's a topic that you don't think is important, even if it's a topic that no one thinks is important, I think there is still value to exploring that boundary of what we know and testing the limits of our understanding.
So many people spend their lives consuming rather than creating, doing the same things over and over again every day, maybe producing lots of economic value but not doing anything novel or interesting to anyone. A PhD forces you to do something different.
Of course you can do novel things and do deep research without doing a PhD, a PhD is just a certificate to prove you did it. I value the process equally regardless of the result of the research or the setting it's done in. And I don't discredit the value of work that isn't novel either - there's also a lot of value to the routines that make the world function day to day. I just think that in a world where most of us will never get a chance to spend years becoming a true expert on something obscure and novel, that encouraging more people to take that opportunity and explore that niche is worthwhile.
Obviously, that's only one avenue for talent. Some talented people never do a PhD, they may create start-ups etc instead. But its about fostering an ecosystem to develop and retain talent.