Since you have to reclaim your checked luggage to go through customs, you're no longer in a "sterile" part of the terminal. You might have had some items that are not allowed on board in your checked luggage. Therefore, you have to go through security again after rechecking your bags.

IIRC, some countries allow you to clear customs at your final destination so that you don't have to claim and recheck your bag. However, that's not really practical in the U.S., as you would have to have customs facilities at hundreds of airports.

Theoretically, an airport could provide a "hand luggage only" path after exiting the immigration that would lead directly to airside domestic; since you wouldn't have access to your checked luggage, you could still be considered to have been screened. However, I don't think the U.S. is willing to rely on other countries' screenings across the board, so I'd bet TSA would still want to rescreen pax in this case.