It's easier to just tax exchanges. If you cash out, you get taxed - if you hold, nothing happens.
It's nightly impossible to keep a record of every single bitcoin wallet, moreover with single-use wallets and paper wallets.
If you get paid on bitcoins, the tax should be collected on the payout.
The government won't be checking your bank account to get your taxes, it will just tax your transactions and products. It kinda doesn't matter what currency you are using.