Because the security underpinning blockchain is either "spend a bunch of expensive electricity and get extra tokens" (proof of work) or "lock up some tokens for a chance to get more" (proof of stake)
These tokens have to be worth something in order for the security to functionally exist. You can't separate the monetary side from the security, because the monetary side incentives the security. And security is the only thing blockchain adds.
I don't completely agree with that. Yes the network must be secured that way, and the native token must be worth real money, but that's not something applications built on top necessarily need to worry about or interface with. You can build applications that do not use the native token (outside of transaction fees) and that doesn't affect the app's security.
These tokens have to be worth something in order for the security to functionally exist. You can't separate the monetary side from the security, because the monetary side incentives the security. And security is the only thing blockchain adds.