On my first visit to Hollyweird I went to the Tonight Show and The Price is right in 1987 among other events and shows. I was not called down.
They choose those people based on a combination of youth, outward enthusiasm, good disposition, ability to not clam up on TV, and general healthy photogenic appearance.
I tried to get on a few shows. I had none of those traits, but I noticed the choices made had those qualities PLUS an interesting background so the host could make a funny comment about it, or some outstanding physical trait. I remember one contestant had a beautiful smile, seemed almost a mile wide.