Actually I think it's because they don't want to spend the money to create sanitised international departure spaces.

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