Some criminal investigations start with the evidence that a crime has occurred, and links additional evidence to the web that grows out from there, until at least one suspect is linked to a public identity.
If you start by making random accusations against innocent persons in an attempt to generate additional leads, you're not investigating, you're just playing "Clue".
Police resort to unlawful tactics when they have no leads to investigate and political/administrative pressure to close cases. Or when they are rewarded for making large seizures and well-publicized arrests, and are never punished for the sloppy work that led up to them.
The US system is supposed to encourage following the evidence to the suspect, rather than picking a likely suspect and then trying to connect that person to the evidence. "Parallel construction" is a direct attack on the presumption of innocence and personal privacy. It is part of a policing culture that encourages individual police to be lazy and slipshod in their investigations--and that's how you get 2 corpses and injured police at 7815 Harding Street, with some heroin dealers doing an orderly cleanup and evacuation at 7815 Hardy Street, even as the PR flack from the police union and the police chief are busy blaming the victims and doubling down on the botched raid. That's how flashbangs land in a baby's crib. That's how the family dog gets shot, no matter what.
When we do not make police work difficult enough that some investigations fail, one does not need to be diligent, attentive, and dedicated to pursue a career with the police. A person who is violent and sloppy can still get their promotions and pensions. Computer says arrest this guy, so cop goes out to pick him up. The person who is not smarter than the computer cannot recognize the limitations of the system. It becomes a magic box that spits out the right answers--most of the time--without having to do any of the homework.