Why Galileo's thought experiment about the ball rolling forever on a level surface could only be a thought experiment.

Because in reality there are frictional forces acting on the ball, against the direction of its motion. In fact, because of the friction between the ball and the surface, the ball loses little by little its energy, so its velocity decreases and eventually it stops.

In an ideal world with no friction, there would be no forces acting on the ball, so its energy must be conserved and this means that the ball would continue its motion forever.