Yes, Bates is a thing, but not every document acquired during the course of an investigation is responsive in discovery because it may not be relevant, not to be used in trial because it's factually deficient, privileged, someone struck a deal, it's just some poor innocent schmuck caught up by happenstance, etc etc etc. Government is only obligated to produce materials that will be used at trial, establishes provenance for those trial materials, or that may be exculpatory.