The fact that forces must be balanced on a stationary object is a consequence of a more general rule called Newton's First Law, one of three laws of motion discovered by Sir Isaac Newton in the 1600s. The first law says
If the forces on an object are balanced, then the object will maintain a constant velocity; and if an object is maintaining a constant velocity, then the forces on it are balanced.
Strictly speaking, Newton's First Law only holds for objects that are in inertial frames of reference, but we'll have to wait until later to explain what that means. You can assume that it always applies until I say differently.
Because a stationary object had a constant velocity of zero, the forces on it must be balanced, as we've already seen. But Newton’s First Law also says that a car going at 30m/s down a straight highway also experiences balanced forces.
Notice that there's a big difference between constant velocity and constant speed. A car going around a curve might have a constant speed of 30m/s, but it cannot have a constant velocity, because its direction is changing, which means that the forces on this car are not balanced. We will look at this example when we talk about centripetal forces.