First of all, there are institutional and personal checks in the system to discourage or prevent this kind of behavior. They're not always as effective as we might like, but they're there. There are also ways for defense attorneys to challenge this if it happens. Perhaps most importantly, there's usually no need to invent a confession, or force one: people will confess on their own with fairly minimal pressure.

Second, it's very unlikely to happen often. Here's a useful anecdote: I had a professor who used to be an employee of the DOJ, and most or all of her job was prosecuting dirty cops. She told us about a case she had against a cop who'd pull people over, rob them of any drugs or money they had in their car, send them on their way (or occasionally arrest them), and split the profits with his co-conspirators. She noticed that every time he wanted to pull someone over, he waited until they violated a traffic law that gave him probable cause, as the Fourth Amendment requires. She asked, 'why do you even bother sticking to the rules on traffic stops? You're stealing drugs from these people!' And he looked at her like she was crazy and said, 'Well, I'm still a cop.'

That guy knew he was a criminal, obviously. But he also thought of himself as a cop first and foremost. I'm not saying that the rules on traffic stops are sacred, of course. The lesson is that even outright dirty cops (and that guy, counting various other crimes, was about as dirty as they get) still think of themselves as police. And outright fabricating a confession, or actually holding a gun to a suspect's head to make them confess, would flatly contradict that self-image. If only because it's such a complete failure of investigation and actual police work that it makes the cop look so shitty at his job that he can't even get a confession.

Finally, I'm sorry to take this out on you, but I hate these kinds of comments. Some redditors get really fixated on the police credibility advantage, extrapolate that into "police can lie on the stand with impunity," and extrapolate that into "police can do whatever they want, because they can just lie and claim they weren't breaking any laws, and everyone will believe them." And it's really frustrating. First of all, claiming that police are regularly lying through their teeth to force convictions is a major claim. It thus requires a lot of proof. No one I've come across has been able to come close to satisfying that burden of proof.

But what really irritates me is that it's essentially impossible to argue against, and not because it's such a great argument. Eventually you have to throw your hands up and say, "fine, you're right, it's true: a completely immoral cop could do all this." And then the response is, well, what the fuck do you want to do differently? What rules could you possibly put in place that would completely stop police from lying about such things and being believed? It's only a problem if you have a solution. Then there's a historical fact we need to keep in mind, which is that this used to be much worse. Police used to have basically zero oversight, and before the Warren Court, criminal defendants had a lot less protection.

More fundamentally, the assumption that the vast majority of police are trying to do their jobs and won't flagrantly and intentionally violate the law is something so close to the heart of the criminal justice system that, once you throw out that assumption, it's hard to have any discussion at all. It's as if we were trying to have a debate about what the defense budget should be and you were like, "but there's nothing to stop the military from mutinying and forcing Congress at gunpoint to vote for a higher defense budget!" It's divorced from reality, it speaks to your paranoia much more than actual flaws in the system, it brazenly reorients the terms of the argument to the point where not only is it an entirely different argument, it's hard to have any argument at all, because if things fall apart to that degree we'd all have much more to worry about than accurate trial testimony, or the size of the defense budget.