Becouse to create crypto or btc you need electricity.
Just to correct you:
To create
NEW bitcoin blocks you need electricity and there is already around 18.6 million of BTC in circulation while there is only 2.4 more million to be mined before we hit a hard cap of 21 million.
There doesn't have to be anything backing the BTC. It's fundamentals are enough; the way it works is already better than anything else we previously had as currency.
Also, I am not sure you understand how traditional currency (fiat) is made, what is it based on and backed by either.
As this is in Bitcoin part of forum. I won't comment about PoW and PoS difference and other altcoins which you also mentioned and referred to as "crypto".