I have a counter-question for you. Why does a loaf of bread costs less than a diamond despite the fact that a loaf of bread has a higher utility value since it allows humans to eat and survive, whereas diamonds do nothing useful to humans?
I gotta interfere:
Uses of a Diamond:
Diamond is considered to be the hardest non-metal present on earth and is used to cut metals and glass. Diamond-tipped tools are used to cut or drill various objects. Diamonds are also used to heat the sinks that are used to conduct the heat apart from the sensitive parts of the microelectronics.
A loaf of bread is more useful than diamond for humans probably but it is also not as rare. Diamond is rare af and it is far from being useless.