Can someone explain, in small, grampa-compatible words, why is bitcoin un-taxeable?

I mean, I am an old, possibly senile, Canadian, and maybe I am missing something big, but... I recieve payment to single-use addresses, but I pay my gas, my taxes, my food, and pretty much everything else in fiat, so I exchange btc > fiat (and since I want money to go to my bank account, I do that in my very own name)

Obviously, government can detect income at this point, and tax me (also possibly audit me because I have this very strange income structure going)

Even if I could buy stuff with BTC, if I want to stay below tax radar, I would only be able to buy small-time stuff, like groceries and occasional small items, because as soon as I buy something huge (like a house) with my undeclared BTC riches, tax folks can come down on me like a blivet, right ?

So... where does the "cannot be taxed" idea come from, again?

How is getting BTC different from getting small-denomination worn cash?