Turbulence too. With a flat surface, there's no way to anchor to a physical point before pressing. With buttons, there's tactile feedback so you can keep your finger on the button through turbulence until you're certain it's the one you want to press.
My experience even in older cars (where bumps more likely affect your control efforts) is that you also need fixed perches near the important controls. Then, you can rest part of your hand on this perch while aiming fingers to the controls.
I think avionics soft buttons are usually arranged this way, and one can imagine similar touch interfaces that would focus on the edges of screens with robust bezels of some kind.
Your body and arm might be shifting around due to the shaking of the cabin. Your hand is stabilized relative to the buttons or knobs near the perch, so you can aim precisely.
What you don't want under shaky situations are controls way out in the middle of a field of controls where you need to aim precisely and have no perch for your hand. Even worse would be the kind of consumer touch UX with drag or other gestures where you could accidentally input some high magnitude command because a cabin movement pushed your free-aiming hand across the screen...
