It is in the best interest of the passengers (at least the international ones) to go back to the airport and get the immigration done. It will only cause trouble at the time of future departures/arrivals, and cause issue in visa extensions, etc.
That means the US wouldn't always have a record of the exit date.
1. Many older terminal buildings were designed without any space for exit controls. By the time they realized they would need exit controls (relatively recently) they realized they could just slurp the data from the airlines’ systems instead.
2. Immigration is chronically understaffed. The government would need to double the number of employees, which is never going to happen.
It's a mistake that means that one can't transit through the US without passing through US immigration (which typically takes hours). Hong Kong which I pass through a lot is more like a train station, you can get off of one plane and onto another in an hour or two, just have to pass through a bag check. This means that non-US people choose to fly through other countries, on other countrys' airlines just because the US is an impassible bloc for many