It is like with all laws. There isn't an agent following your every step telling you what to do and what not to do. You are supposed to know the law and abide b

It is like with all laws. There isn't an agent following your every step telling you what to do and what not to do. You are supposed to know the law and abide by it, and they will occasionally have people make sure of it, be it by police or tax auditors. For taxes, how is the government supposed to know about the work you did in exchange for cash? Or make a difference between a vacation and a tax destructible business trip. They will sometimes look into it if they suspect fraud, but they have no way of knowing exactly what you owe beforehand, and that's a good thing, surveillance is as bad as it is right now, not mentioning the costs. The best they can do, and that's how it is done in most countries, is to tell you "according to what we know, you owe us XXX", but it only works in the simple cases, like for those whose only income is their salary. If they missed something and their estimation is wrong, you must declare it or you can be in trouble. It is just a convenience, you still need to do your taxes correctly and you are responsible for it. One may argue the rules could be simpler, but they are complex because the world is complex, and even small changes can make or break industries.