Well, they know at the end.

A classic example would be if you take a take a deduction for auto-expenses or mileage, the government won’t know how much less to take if you don’t calculate the deduction at the end.

Or let’s say you don’t take the standard ~$13k deduction but instead want to do itemized, they have no way of knowing your itemized deductions unless we were to go full tilt with the government having a profile for every person and tracking every single transaction they make.

Or, you don’t need to contribute any more to Social Security once you hit a certain amount, but you have to contribute a higher % to Medicare once you hit a certain amount; if you have more than 1 job, how would either job know how much you are making in total (including any other sources of income)?

If US taxes were simplified, then yeah, having each worker file every year would not be needed.