I suspect a lot of it is to ensure people who are paid in cash have a strong moment where it can be argued that they took significant agency to hide that from the IRS.

“I just clicked yes on the web form.” might not feel as compelling to a jury filling out paper forms and conveniently forgetting my cash-based income. (I suspect the same reason is why you have to carry your luggage a few hundred feet over a line and recheck it when connecting on an international to domestic flight coming into the US: so there is a clear moment when you personally carried your luggage over a line, so if there’s material that wasn’t allowed to come over that line, you’re attached to it when it happened.)