Because taxes are for the expenses of the government to fulfill their duties for the people. Even though we do not agree to pay them or take it as a burden, we do not have a choice but to support and pay our taxes properly.
So, in fact, we have no way out. Such a puzzle without a break. But already in corrupt countries there are ways to get around it and not pay.
It is irrelevant to pay tax on your Bitcoins because these are not asset and has no physical appearance and furthermore it has nothing to do with the government so tax can not be implemented over it. Bitcoin is a currency like other currencies but is decentralized in nature so we all should keep in mind that tax is only possible if we buy a house or any place with Bitcoins profit.
There is a stamp duty or purchase tax on houses so I tend to agree with your idea in theory. However most governments want to tax purchases, tax the ownership annually of a house or certain items, charge licence fees and in addition charge a tax on earning any income. As well as that most governments dont like anyone gaining money on an asset and so Bitcoin is considered an asset and is taxable to them.
Usually they send the army if you dont pay taxes in large enough amounts to seize every asset they can, or small amounts they might put you in jail. Unfortunately even when right the government doesnt give an option for you the public in paying their bills