Mostly it's because there's almost zero isolation between tracks. About the only thing(s) that can be isolated are those that have direct ins. Anything that's mic-ed is going to include at least a little bit of everything, including instruments, vocals, crowd, stuff getting dropped backstage, etc. Every mic pics up everything, at least to some degree.
"I spent ten years starving to death playing great music. I write a one-chord song about poontang and make a million dollars. What would YOU do?" - Ted Nugent