I'm sure it's possible to use servo motors to get precise control of the ailerons, elevators or the rudder. No, it isn't. Primary flight control actuators require both rather high forces and quick response time. Electric motors can provide either, but both at the same time is a problem. read more
They do use electric motors where they (electric motors) would be the most efficient, and conversely, use hydraulics where the work load requires greater “bang for the buck”. Each one’s benefit is well known by now. read more
Planes use hydraulics because of the immense pressures on the control surfaces during flight. Hydraulic systems can deal with higher loads than motors of similar size. An electric motor can get "stuck" when it encounters a load that is greater than it can move, possibly causing a crash if it happened in an airplane. read more
There are several advantages of pneumatic (compressed air) over hydraulic (pressurised liquid) or fueldraulic (pressurised fuel) systems: No need to carry hydraulic liquid. That saves weight and maintenance cost. read more