It’s simple. This is because everyone believes it is money and values 100USD.
In the ancient time, goods were used as money because they had the same value including gold.
Later on, when people started to use paper money, it wasn’t just paper but was guaranteed that you could exchange it to the same amount of gold or silver.
However, our money today is just paper and you cannot exchange it to gold or silver, but people believe it has value. That is why everyone uses paper money.
In the world, there are some systems that make you believe the value of money. For example, only the central bank is given the sole right to print currency to control volume of currency and credit.