Most cops don’t do detective work because that isn’t their job. Most cops do grunt work like tickets, traffic, taking down statements and reports, etc. Detectives do the detective work.
Even if they did do good work, the prosecutor has to follow through with filing charges. The cop can get all the details but if the prosecutor doesn’t think they can win or get a plea then it doesn’t even matter what the cop does.
Which then circles back to why there are so many lazy and corrupt cops. They are only incentivized to do either the easy stuff like writing tickets or the high profile (rich people as victims) stuff because that is what the prosecutor follows up on. The prosecutor doesn’t want to hold cops responsible, because that is where the stuff they need for prosecuting people comes from.
Which is why we have lazy and corrupt cops. The system incentivizes both.