"Not moving" and "not accelerating" are different things — and treating them as the same is one of the most common errors in engineering mechanics.
Say "equilibrium" to most people and they picture something sitting still. In engineering mechanics, that picture is only half right. Equilibrium doesn't mean a body has zero velocity — it means a body has zero acceleration. A block sitting motionless on a table qualifies. So does a car cruising down a straight highway at a rock-steady 60 mph. Both satisfy the exact same governing equations, for the exact same reason, even though one of them is moving and the other isn't.