Actually they occasionally are used. Case in point the schizoid Mexican who shot up the NG members in a Carson City NV IHOP. Fully select fire. Very good smithing job. Sold in California 5 years prior. They attributed the inability to trace it due to non regulation of PPT sales in NV but it was sold in Cali and any PPT without a FFL involved in Cali is illegal plus I will bet the weapon was never registered when Cali had its AW registered.
That being said it was a Norinco MAK 90 and I also believe what isn't being said is that the reason a trace is impossible is because the AK wasn't serialized.
Look up Operation Dragon Fire where the the PRC were running non serialized select fire AK’s and SKS into the U.S. Norinco and Polypro were the main conduit companies and who was their US representative at the time none other than Dick Blum the hubby of “turn im all in” DiFi of Senatorial fame.