So what are dollars backed by? The "full faith and credit" of a government, which can simply poof them into existence. In the case of US Dollars, this has worked reasonably well. In the case of Zimbabwean Dollars, not so much.
Maybe you want to go back to a Gold Standard, where the bills are "backed" by vaults full of metal. Then what is gold backed by? It's turtles all the way down.
UItimately, the only reason Bitcoin has value is the same reason USD, bars of gold, or anything else has "value": because some critical mass of people agree that it does.
To quote Warren Buffet:
"[Gold] gets dug out of the ground in Africa, or someplace. Then we melt it down, dig another hole, bury it again and pay people to stand around guarding it. It has no utility. Anyone watching from Mars would be scratching their head."