Because, to many people, 30 is the beginning of real adulthood. It is the age when most people seem to think that you should have your life in order: i.e., gainfully employed, married, having children, living in a house or apartment (away from your parents) et cetera. It is an artificial, yet perfectly valid milestone for many people.
I mean, the prevailing notion is that if you are unemployed/under employed, single, childless, and still living with your parents at the age of 30, the either you cannot or you will not grow up and take responsibility for yourself.
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The mere fact that science may not yet adequately explain an object, event, or experience does not mean the immediate explanation should automatically default to a conspiratorial, extraterrestrial, paranormal, or supernatural cause.