AIA parking is strictly for those who want the convenience of parking as close as possible to the terminal. All about demand and supply, and convenience.
There's actually plenty of parking around the area (e.g. Aeroparks charges $30 for first day, $33 for 2 days and $39 for 3 days etc, cheaper as it goes for more and more days.
A shuttle bus takes users to and fro the carpark to the terminal every 10 minutes - and the terminal is but 5 minutes away.