Because they are supposed to possess minimum levels of intelligence found in homo sapiens, which includes not believing anything a salesperson says.
Also, their whole job is dealing with people who constantly lie to them.
Some police departments seem to actively reject candidates that have higher scores on IQ tests. Not that I think IQ test scores and actual intelligence are related but it clearly shows their intended target candidate group.
https://abcnews.com/US/court-oks-barring-high-iqs-cops/story...
You're over-selling the minimum level of intelligence in homo sapiens.
What you're stating is your wishful thinking. Don't get me wrong. I'd also like what you say to be true. It very much is not. Quite the opposite, which is why salespeople "work".
The amount of AI bullshit Senior+ level developers just paste to me as truth is astonishing.
There are two things occurring here.
Police get raises and recognition for closing cases. In general they don't care if you're guilty or not, that's someone else's problem. Same with the detective, same with the DA. The more cases they close they 'tougher they are on crime'.
The next thing occurring is
https://en.wikipedia.org/wiki/Computer_says_no
https://en.wikipedia.org/wiki/Computer_says_no