> the government knows how much I make already

Taxes aren't based only on what you earn. They're also based on your filing status for that year, how many dependents you have, account interest earned that didn't generate a form (IIRC most banks don't send this unless it's over $10), money you gifted over the annual exclusion limit (not taxed until you use up your lifetime limit, but the IRS doesn't know about this until it happens), how much you contributed to IRAs (any forms generated for this happens after taxes close out), how much you contributed to HSAs (other than through your employer), etc., etc.

Point is they may know how much you make, but until they know everything above and more they don't know how much you owe.

Could they send everyone an estimated tax return? Yes. Would that eliminate the need for people to scrutinize their work and correct any errors? No.