Together with some major bills for insurance and healthcare, that's all they get.
So I can hide cash under the bed, or have an overseas account, or have some Gold bars, etc. The goverment has to thrust me quite a bit.
They can check some transfers, but if you like to hide it, you can do it.
So the actual way of getting the real numbers is quite bad.
In Bitcoin
If the Tax department makes it a rule by law, that you have to report every BTC address you own, or every wallet you own ...
All problems solved, because then the Department has also the addresses of the petrol station and the deli you spend money.
In no time the system knows which tax counts here and can claim the respective amount from both sides.
Sure it is still possible to hide Bitcoins, but then it is illegal.
...or did I miss something?
People who will want to evade taxes will just not report all their addresses. They will keep coins on separate wallets, use multiple accounts on exchanges and other services, run their coins through mixers, exchange Bitcoins for some anonymous alts like Monero, Zcash and Dash, and then convert them back to BTC. If you are not dumb, you can hide your coins well enough. But of course noobs will be getting caught, like they are getting caught today on this forum, when they have multiple accounts and try to join signature campaigns, and when managers check their addresses, they get proof that multiple accounts belong to a single person.