Once I got to the level where I was in SAG, AFTRA and AEA (the unions representing professional Actors in the US), and had an Agent, I moved on to a different class of audition. Before that it was cattle calls like American Idol where the public was invited more or less to particpate in a publicity stunt for the project.
Once I got past that point and had my profesionnal credentials, I was invited to audition in casting offices one-on-one with professionals in the room. Now, the down side of that is that you get called in with 20-30 other professionals who are as good, if not better than you. So, the casting people have a luxury of making a choice based on something other than if the performer knows how to do the work. So, the odds, even for a professional are slim at any given audition.
I suppose that with something like Idol, since the pro's do rise to the top of the heap, maybe they get a better chance than they might working through the standard industry. Next to all those high school talent show rejects, they end up looking really good, where next to their peers they might appear just average.
The odds are still staggering when you look at those stadiums full of people wanting to try out. If I were looking for work right now, I'd never go near anything like that.