There are a bunch of reasons evidence would potentially not be admissible, and I'm not a lawyer.
But my point is that you're arguing these rules shouldn't count in cases where you "just know someone is guilty." That's counter to literally the most basic principle of our legal system (presumption of innocence).
Innocent until proven guilty. That being said, a lot of verdicts depend more on the skills of the attorneys and less on the actual guilt or innocence of the defendants. It's all a game, and like in most games, those with more money can pay their way to a higher score.