Those guys do come on the show, and very rarely win.

The winners are most often low-profile professional magicians who make a significant income consulting for the famous ones. Inventing and selling tricks is an important revenue stream for many magicians.

So they come on, do some trick that they invented in secret and worked on for years, and give P&T that same childlike sense of awe and amazement that the rest of us experience almost all the time. It's the best part of the show.

Like this guy: https://www.youtube.com/watch?v=LyZp4sPqBAI