Because a lot of events and processes in history made them
But... do we really us paper so iften right now? Now, if I'm using fiat money, it's always in digital form. I have almost forgot how it is to have this papers with Important People Of Your Country in my wallet. And this change had happen in one decade! And now, we're slowlly switching to crypto (in some countries it has happen already, in Ohio you will be soon able to
pay your taxes with bitcoin), and despite different form, it has the same role - to maintain the system, to keep economy stable. The spice must flow! The change from credit cards to credit cards based on bitcoin won't be so radical as change from barter to fiat money. It'll be a process, an in its consequence we'll only change a form of paying, not base of economey itself.