State economies are mostly based on FIRST transactions, leaving only tiny space for other currencies. If Bitcoin transactions were to be more accessible and widespread there would perhaps be interest in making them taxable. But in most countries, taxing BTC transactions depends solely on the honesty of the parties involved. So that's not that much of a benefit for a country's economy.
I don´t know exactly how it works, I guess it depends of the country. It´s true that goverments can´t tax bitcoin transactions but they can tax purchases made with bitcoin. For example, a company that accepts bitcoin makes a sale with this currency that they have to declare to the tax authority, the company would be taxed for this sale, they wouldn´t pay with bitcion but with the local currency. There are APIs that allow merchants to make sales with bitcoin and those coins are converted automatically to their local currency. It´s one example. At the end bitcoin is money, if you create an enviroment where people use bitcoin everyday it will will bring movement of money and economic activity.
