Car appreciation... the aesthetics, the engineering, etc

Could be something to do with signal interference?

Cost, complexity, weight.
I would imagine the potential for failure of a pneumatic valve actuator is a few orders of magnitude lower than for an electrical one. Pneumatic valves are proven in F1.

Electric ones would require linear motors, electrical connections, etc, right in the hot, wet explosive part.
Pneumatic channels, and possibly the actuator bodies themselves can be cast right into the head.

I've been doing some reading and it's what BlueRinse says.