Marketing: What market do you really want?

Who is your target market?

If you said everyone, I suggest that you look at the most successful businesses.  Even they don't have "everyone" as their market.

I am not providing any true insight here, as many people have said this before me (and you can bet that plenty more people will say it after I do, too).

However, the lesson must not be clear enough, because I hear both beginning and established businesses make this mistake often, like nearly all of the time.  (I did not say that ALL businesses do this, only that I can find a business guilty of this nearly every day, if I try.)

So...Why is it unwise to target everyone?