Because I am doing a whole lot more than what can fit on an Arduino shield. My design allows me to plug the FRDM into my board and the board has 5 chips that convert 30 IO signals from 3.3V to 5V and back and then also provides necessary current to drive a dozen or more high current 24V IC switches which then drive hydraulic valves. I looked and there just ain't enough space on them thar lil bitty Ardweeno boards. :-)
I have the board built and am looking forward to testing it as soon as I can get my Android tablet communicating to the Freedom board so it can control my new interface board.
The FRDM-Android USB interface is not being as cooperative as I would like it to be this Christmas season. :-(