Some people can't sing, but they do. It's an unfortunate phenomenon that mostly happens in their cars with the music turned up just enough so that it can trick their ear and their minds into thinking that they can sing. They go along with the trick of the ear because they like the feeling of being able to sound like their favorite singer. They can pretend that they are talented, and famous, and beautiful.
Sometimes they trick themselves so completely, that they can start to do it without the music turned up. Suddenly they think they should be on national television competing to become the next American Idol. Let's be clear. This is a mental disorder. It even has a name. It's the Dunning-Kruger effect.
The Dunning-Kruger effect is an example of cognitive bias in which "...people reach erroneous conclusions and make unfortunate choices but their incompetence robs them of the metacognitive ability to realize it". They therefore suffer an illusory superiority, rating their own ability as above average. This leads to a perverse result where people with less competence will rate their ability more highly than people with relatively more competence.
American Idol is a show that puts this widespread disorder on display for all to see.
I am also guessing that there's a version of this disorder that effects the judges themselves, namely Paula Abdul, whereby they think they are sober and coherent while they are stoned out of their heads talking total gibberish.