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?