First, yes, we calculate our own income tax, but the IRS also calculates it separately, and if the two disagree, they selectively decide whether to come knocking. (For example, they once tried to bill me $300,000 for a tax year when my net income was just over $100,000, because of multiple clerical errors -- we had sold a house, and they had both erroneously tried to apply capital gains tax where it didn't apply at all, and tried to tax us for the full sale price of the house as though we had bought it for $0 and flipped it. It took months of back and forth to fix the issue, despite everything having been clearly documented.)
Second, calculating tax liability is just the first step. Actually submitting tax returns electronically required a third party for a long time, and I believe it still does for all but the most straightforward returns. There's absolutely no reason for that to be the case, other than the outsize influence of lobbying money.
That's without getting into all the loopholes, dodges, and hand-waving that make it possible for someone like Trump to avoid paying taxes entirely most years, while those of us who they know can't afford to lawyer up are the ones they try to collect from.
