Before you can really trust another criminal, you have to make sure they commit a crime with you. Rob someone, beat someone, kill someone, etc. To a lesser extent, maybe just use drugs with you. You can't let them have a pass on it, or they might be a cop or some other kind of spy.
Politicians work the same way, and parties control this. To be eligible for president, you first have to be a governor or congressman, so that you have a record of having previously sold your soul. They have to watch you commit heinous, immoral acts, like signing off on a state budget, or shelving a clemency plea, or otherwise playing ball. Without that, they might accidentally let someone good through the system.
Every power-centered industry protects itself this way. In American law, for example, it's possible, though extremely difficult, for someone to fake their way into the position of a judge. They make it tough: first, they want to see that you've been a prosecutor or a public defender. They want to see you as a respectable prosecutor, blackmailing people convicted of victimless crimes, ruthlessly destroying peasants left and right, and ignore-enabling the silent thugs behind the curtain. Almost all judges and politicians are lawyers with backgrounds in prosecution, State Bar administration, professorships, or other policy avenues where they are forced to serially commit mundane atrocities before being allowed to gain influence. Those with any conscience are driven out in the early stages, before they've had an opportunity to use their influence to exonerate more than a small handful of people, and it takes decades of work even to get that power.
A good, decent person could (theoretically) fake her or his way into a judgeship merely by being a social asshole and billing major corporate clients, but a catch awaits: normal judges aren't allowed to make law. Only American appellate courts can "make" law, in the sense that their opinions are binding upon future court decisions (even though they can then be overruled by higher state or federal appellate or supreme courts, if a given judge has a late-life splurge of conscience). Mere "trial" judges are apprenticed for appellate and supreme roles by being forced to commit terrible crimes against people, destroying lives and stealing property. Only when enough horrors have been committed, and the person's goodness has been completely forfeited, can they be trusted to have an effect on policy.
It would be possible to lie and fake your way past these people by only acting like a terrible person if not for the requirement of actually being one, by sentencing people for possession or propping up horrid landowners or finagling a chemical company's way past already-farcical regulations; they vet you like this in order to make certain that you commit terrible acts as part of the process, so that you can't be a decent person who lies in the service of gaining power which can later be used to change the world. No, that power is carefully denied until they've watched you execute a few POWs first. They want to see your soul scarred and bleeding before you're allowed to command formal respect. If you're First Lieutenant, you have to sign off on patrol missions, tell the Privates to go into that neighborhood and smash it, and if you don't, you'll never become Colonel, certainly General. If you're Private, you have to be part of wiping out that neighborhood before you can become Sergeant-Major and be perceived by the public as trustworthy enough to comment on policy.
You don't get to run the police station until you've shown that you're committed, as a beat cop, to evicting the poor, using beatings or murder to defend owners' food dumpsters against the intrusion of starving homeless, and writing traffic revenue tickets. Trying to change the system from within is impossible, because the system is designed to replicate itself by digesting only those who perpetuate it. Once you've done the terrible things necessary to become powerful, you have become everything you might once have thought you were planning to fight against. There can be no sleeper agents.
Physicians are tested throughout medical school, residency, and then their early practice years, vetted away from professorships or licensing committees until they've shown their loyalty to the organization by prescribing and/or condoning lots of chemo and methylphenidate. Journalists don't make editor until they've flushed their young lives demonstrating that they're willing to write fluff pieces and downplay substance. You don't get in front of a live camera, as a news correspondent or an actor, until you've wasted years of your life condoning terror and saying nothing valuable, and proven that you can be trusted not to suddenly look at the camera and tell 40 million people that it's all a lie.