I'm 6-feet tall and even I find airplane seats too high. My feet barely touch the ground when I sit upright. Why can't they make the seat cushion as high as a living room couch?
The short answer,
Your couch is made for you to fall asleep on, watch football from, and generally be of any non determinant shape since no couch (as far as I know) carries a type certificate.
Most pilot seats are in some way adjustable up/down and forward/back depending on the age of your aircraft.