Where I'm living, the government does the taxes for you and then sends you a notice to check them.
That last bit? That's important - you get to check them. You can adjust if you should need to. Guess what most folks don't have to do? Check them. This works because most folks have simple returns. They work, might or might not own a house, might have a bank account, might have a retirement account - and that's about it. The government already has what it needs for most people. But you can check them and adjust if they are wrong. Of course, you can just not bother, too.
Is the system perfect? No, but it doesn't need to be. It merely needs to have better overall tax compliance than another system. When taxes are easy for the vast majority of your population, the governement has a better collection rate and can spend manpower auditing those that need audited (perhaps audit more complicated returns from the well-to-do instead of Ordinary Married Citizen that makes 50k jointly) and they can generally spend less money doing things like printing forms and manning question lines that don't give answers that constitute legal advice.
And seriously, in these systems, hiding income is still a crime. So those folks that just "used the IRS figures!" still have the chance of going to jail. Even if they never get caught, the automated system still wins because of the other benefits.
That last bit? That's important - you get to check them. You can adjust if you should need to. Guess what most folks don't have to do? Check them. This works because most folks have simple returns. They work, might or might not own a house, might have a bank account, might have a retirement account - and that's about it. The government already has what it needs for most people. But you can check them and adjust if they are wrong. Of course, you can just not bother, too.
Is the system perfect? No, but it doesn't need to be. It merely needs to have better overall tax compliance than another system. When taxes are easy for the vast majority of your population, the governement has a better collection rate and can spend manpower auditing those that need audited (perhaps audit more complicated returns from the well-to-do instead of Ordinary Married Citizen that makes 50k jointly) and they can generally spend less money doing things like printing forms and manning question lines that don't give answers that constitute legal advice.
And seriously, in these systems, hiding income is still a crime. So those folks that just "used the IRS figures!" still have the chance of going to jail. Even if they never get caught, the automated system still wins because of the other benefits.