The only reason for this is Newton's First Law which states that :- An object continues to be in its state of rest or uniform motion until and unless and external and unbalanced force acts on it.
actually force acts on every object that exist in this universe but when an object moves in a friction-less surface with no acceleration i.e. with constant velocity then no unbalanced force is required to maintain it's constant velocity and i can say this from the fact that ''an object at rest and an object in uniform motion continues to maintain it's state of rest and state of uniform motion unless and until an unbalanced force acts on it ''( newton 1st law of motion ).

i hope it helps...............