They don't hold significant gold or reserves to back them up. Not since the end of the gold standard decades ago anyway.
They're all "fiat" currencies unrelated to anything to back them up in reality.
The likes of the US$ and € are large reserve currencies and just effectively are valuable because they are what they are - accepted units of exchange / value storage.
Legal currencies are regulated and will be backed by central bank interventions though.
The differences between virtual and traditional currency is less than many people think though!