Two points:
I'm thinking preclearance countries are not drug-source countries and the preclearance photo process doesn't not scale.

Bag from drug sourced country goes to NNN-MIA-ORD. Courier gets off at MIA and out the door without regard to status of bag. Bag goes to ORD without passenger since this is not international. Someone else pickup bag at ORD. I'll be surprised if CBP ok with that and willing to let go of current process.

If we consider photo clearance it (currently) doesn't scale well. Pre-clearance bags at originating pre-clearance airports are checked-in to the system over a course of several hours. We're now talking about doing this for several hundred bags in within a short time from flight arrival.

Taking Canada preclerance connecting flights as an example. At peak times, connection passengers have been held >40 minutes at CBP preclearance to wait for the flight's bags to be "photoed". So instead of waiting within baggage inspection area to claim bags, you now have to wait in primary inspection area for bags to be photoed before clearing primary. Not saying it can't be done. Just that there is not much benefit for passenger (from a time saving perspective). Under certain scenarios, this could actually add more time to the overall inspection because you can't even join the primary inspection queue until the flight has been "cleared" after all connection bags are photoed.