Because simply saying something is never good enough. People just say shit all the time, where a court has a responsibility to actually try to find the truth.

Think about a murder case. Should you release everyone that simply says they didn't do it, or should the court look for more evidence of their innocence?

It's a messy process because it has to be. Historically, we used to use even sillier methods, like trial by combat and such. Just your words alone has never really been good enough though, because people can just say stuff.

Even when the things they're saying "sound" reasonable, that's still not good enough.