Why do airports in the US do this? At other airports in other countries your bag moves automatically onto the new flight. In the US I have to manually pick it up, almost leave the airport, and then go back in again to drop it off?!
Because this is not about this case
You’re taking a few examples (try something other than Newark next time) and overgeneralizing.
Not every airport in the US has customs, if you arrive at an international airport you need to go through customs and then travel anywhere else.
The US has agreements with specific airports in some countries like Canada to allow them to skip that step by going through US customs in another country.
For example international transfers in a major airport like Frankfurt you do customs but you don’t see you bag and you don’t basically leave the airport.
In Newark you go all the way out to where people are being dropped off for departures at the curb!
You do not have to do this at most US airports. New York City and Newark in particular is a special case of bullshit.. I avoid that place at almost all costs. There’s a reason those tickets are cheaper.
Because a customs check is not complete if they can't also check your bag on demand?
Wait, really? I've yet to not get pre-check when flying internationally since I signed up for Global Entry, but I don't know if that's just luck.