When I visited the US I was surprised that there's no passport control on departure. Like, at all. I even thought we somehow went wrong way when we got to the gates after TSA, but nope. We asked an employee and were told that that's the way it is in the US. UK is also like that.
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.
That means the US wouldn't always have a record of the exit date.