Because they are not owed any explanation. It's our money, we should be able to spend it on whatever we want. We don't have to prove anything to anyone.
Anything else means the money doesn't actually belong to us, it belongs to the bank or government and they're just generously allowing us access to it. People work hard every day for a living, they shouldn't have to suffer this sort of indignity.
You're born into a contract with society. If it's purely "your" money, then you should exist outside the protections offered to you by society that is based on a currency anyway.
Contracts are voluntary agreements. Nobody is born into a contract. The idea of a social contract implies we have a choice: accept the group's rules or be removed and lose group benefits. This is simply not possible.
We are forced to try and change the system from within. The means for doing it vary. Lobbying the government, electing favorable politicians, violent revolution... In current times people can just build technology that implements whatever change they want to see in the world. Technology so powerful it alters the "social contract" whether governments want it or not.
Sure, you can destabilize the contract. Make your own money that has its own pitfalls. Just because it’s more advanced technically doesn’t mean it’s better for a society.
A deflationary currency is actually terrible for a world with population growth. It ensures if you were born earlier you have more wealth.
Anything else means the money doesn't actually belong to us, it belongs to the bank or government and they're just generously allowing us access to it. People work hard every day for a living, they shouldn't have to suffer this sort of indignity.