Been over this countless times just in the short while I have been on the board. Bitcoin is backed by the strength and security of the network as well as the safety compared to fiat. Ultimately it all comes down to trust, even with government "backed" (in that it is not actually backed by anything physical) paper money.
You confuse BACKING with SECURITY, Backing a Currency means giving it value, government legal tender laws and the ability to pay your taxes in a currency, or in the past pegging the currency unit to quantity of precious metal all BACK the currency, aka they give it some minimum value and allow it to act as a store of value. Security is preventing a currency from being stolen or counterfeited and governments also generally provide Security both by making paper bills hard to counterfeit and punishing thieves, the nature of paper-bills as an bearer-bond put limits on how secure such a currency can ultimately be though.
The BTC computer network provides SECURITY only, it keeps BTC from being counterfeited, and it kind-of prevents bogus transactions by cryptographic signatures, while turning a blind eye to scams, hacked wallets and black-markets making the security arguably inferior to that provided by government (note that all the prosecution of theft/scam under BTC is in fact being done by Government, so BTC security is effectively being subsidized now). But all of these things are just guaranteeing your continued ownership of BTCs not that the coins will be WORTH anything. BTC is semi-secured but not backed by anything.