It's much cheaper to say "confess your crimes or face much heavier penalties". It worked for the French Revolution, the Soviets and the Inquisition, why shouldn't it work in the USA?
Besides, the prosecutors are pretty even handed. They wouldn't prosecute someone if they didn't think they deserved a stiff penalty. As Carmen Ortiz says; "This is an adversarial system. We're here to prosecute."
[I think you're being sarcastic, but...]
The converse, plea-bargaining making prosecution easy and profitable (see seizure law), has resulted in huge expansion of laws making ordinary activities illegal.
If it was hard and expensive then we wouldn't outlaw behavior every time some twit get's their panties' in a twist. And we wouldn't automatically bend-over and use the criminal justice system to prop up failing business models.