Because (in my experience) no one ever declares themselves an "indigo child." They are declared an "indigo child" by their mother. Convinced that their child is somehow special, they poke and prod a few medical professionals in vain, only to be told that no, their kid is actually quite normal. Thus they turn to sympathetic ears on the Internet who inform them that their child actually has an odd constellation of supernatural powers and does indeed herald the next step of human evolution. These traits are usually a blend of "Children of the Damned", "Witch Mountain" and "Tomorrow People" which all emphasize the inability of society to deal with the messianic powers of the youth.
Fundamentally, women determine that their kids are "indigo children" because it's not that they're a little odd, it's not that they can't figure out how to discipline them, it's not that they can't figure out strategies to get their homework done, it's that they're homo superior. It's not that they're bad parents, you see, it's that they have the special burden of raising the next stage of evolution.
Why yes. Yes I have met some of these mothers. Why do you ask?