Yeah. That's why the states do not like bitcoin. Because they can not get tax from the money earned with bitcoin. Only when they are converted into cash and transferred to the bank account.
But in the future we can use bitcoin without converting it to money. Then they will not be able to follow in any way.
Even if you use Bitcoins directly (for supermarket purchases, utility bills, buying gasoline.etc), it can be taxed. The government can just ask the retail chain to impose a surcharge on the Bitcoin payments. So if you are paying with BTC, there will be a 2% or 3% surcharge, and this amount will go to the government as tax.
In any case, the 2-3% is less than to pay the taxes in full. I am sure that the government while turning a blind eye is not the use of bitcoins because the volume of use as not affect the economy. While using bitcoins is profitable only to the exchangers.