Because the FBI has pinky promised to develop those and has consistently failed with violations continuing. There’s also plenty of technical reasons why “good internal controls” are a myth. For example, the CIA and NSA could help out the FBI by secretly trawling through that database bypassing all “controls” using exploits / back doors and sending tips from “confidential sources” and parallel reconstruction being used to prosecute.

Btw we also have similar problems with normal police officers misusing records to spy on spouses and whatnot and occasionally we find out about and the department promises stricter controls and reforms and round we go the merry go round.