For the same reason you do not need to pass through security to get into your own personal car. It is a mode of personal transportation, to establish airline style security for GA
is not only an infringment on personal liberties, it has been shown to be cost prohibitive. However, the TSA
contines to push for some form of security in GA
especially for larger aircraft, the aphabet groups (AOPA, NBAA, etc) have and will continue to fight it.
Pilots are idots, who at any given moment will attempt to kill themselves or others.