It's not that we need to use "just paper" as a currency. It was more like it was what we were introduced to a very long time ago. It's been existing for a very long time, so people are used to it, and have been treating it as the "official" currency worldwide. On the other hand, the reason many people don't treat bitcoin or cryptocurrency as an official currency is because even after 10 years of existing, it's still fairly new. Give it more time and they will adjust to it, and hopefully will start treating it as alternative to fiat.