That's because you can double spend good ole fiat. The problem doesn't exist with Bitcoin and most cryptos. There just isn't the need for the same regulation and oversight, especially when anyone can look at the blockchain.
Hm, how exactly do you double spend fiat? I thought that was the problem unique to the virtual currencies.
More than 9/10th's of USD is "virtual". Banks do not have a dollar bill (note, or coin) for every dollar they "have" in their accounts. We're talking at least 90% of the USD has no note or coin to represent it. There just is no reason to have that. Coin and note go through fluctuations where banks try their best to predict how much they'll need, which comes and goes from them.
When Bank A sends money to Bank B, Bank A needs to mark it off their books or they are "double spending" (they could also claim they sent it to two different banks). The US uses clearinghouses as a third party to verify that these things are happening correctly, without creating money out of thin air on the books ("double spending"). This is happening constantly as every single debit and credit card is used, on top of other banking/money service activities.
ON the other hand, any sufficiently decentralized virtual cryptocurrency cannot double spend. That's literally the first thing in the Bitcoin white paper. I highly suggest you read this in order to understand a bit more, and the link at the end of it is very good for a slightly technical understanding.