Human beings aren't exactly rational - if we were, the modern world would cease to exist, and we'd collapse into a Hobbesian state of nature.
Beyond that, currencies are (in a way) also backed to some extent by the value of the goods and services available to be purchased with them.
So cryptocurrencies can have value of the second type, but if they don't have baseline demand then there's little to stop everybody deciding to switch from one cryptocurrency to another at some point in time, leaving the first worthless - whereas baseline demand makes that extremely unlikely for a currency used in any major country's economy.
The alternative - for someone wishing to transact in an alternative currency and not pay taxes - is to pay foreign or private powers for protection services and then simply not pay the tax. This is suicide in the U.S. because we have a large & well-organized military, but is commonplace in many other parts of the world - and indeed, the U.S. is frequently the foreign power supplying arms and accepting its currency for many of these rebel groups.