Velocity can be measured relative to whatever frame of reference you chose. Flight dynamisists are used to working with “body axis” or “earth axis” references.
So the answer is that the air speeds (velocities relative to the bulk air mass) are constant while the ground speeds ( velocities relative to the earth) are not. Kinetic energy is relative to an inertial frame of reference and the V in 0.5*m*V^2 is velocity relative to the earth so the aircraft is being accelerated by the air mass that is moving relative to the ground.