To index to a fiat currency it requires a market maker. Some person or group that will guarantee that if you have $1 they'll give you a unit of currency and if you have a unit of currency they'll give you $1. To meet the demand for this currency it would require the market maker would need to be able to print money, otherwise the price could naturally increase without a way to stop it. Also, to be able to survive a run on the bank, for every unit of currency they issue and take in $1 they would have to keep that $1 in reserve in case everyone tries to cash out at once.

Letting a central authority print money that had no obligations to the general public is dangerous. Also the market maker would likely spend some of the money they take in. If there was ever a run on the bank they would just shut down the currency and keep the money that was paid in, which would make a run on the bank more likely.

This is very similar to online casinos that let you buy chips, but instead of offering gambling services, they would offer annonomous transaction services. Some casinos have just shut down and kept all the money or if it becomes a legal problem (as any solid fully annonomous currency would) could be shut down by their local government.

There is just no good reason to trust a currency that requires a central authority that can print money and that you don't trust and there is no reason to trust that they'll use the printing power responsibly.