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.