Look at it from a different perspective. Why should you assume an object will slow down? The main concept we use to describe an object’s motion is position. We call the rate of change(slope) of position velocity, and we call the rate of change of velocity acceleration. Suppose an object starts at v=0, and we apply a constant force on it to accelerate the object to v=10. So on a graph this looks like a line linearly increasing to 10 and then going straight. Now, if you assume you need a force to increase the velocity up to 10, why wouldn’t you need a force to decrease it? Starting at zero vs starting at v=10 will both be straight lines on a graph. The only way to change velocity is to accelerate it, which requires a force.

Just in case you are confusing velocity with acceleration (very common mistake). Net force = 0 simply means that the object is not changing in velocity. Force is defined as ma, so if f=a=0, there is 0 change in velocity. The problem isn’t that there’s something causing it to keep moving, it’s that nothing is stopping it from moving. You must have a force to change the velocity of an object. Also, all velocity is relative. You could say you’re going 10 m/s. But you could also say everything around you is going at -10 m/s, and you’re going at 0. So how do you stop? You slow down by applying a force so that you move at -10 m/s like everyone else. My point is, velocity being zero is just a convenient starting point, it holds no actual significance because we only care about its change. Whether something is moving or not moving is actually the exact same state. Your velocity is zero on a plane, even though you’re moving. Your velocity is zero on the earth, even though you’re on a rotating object.

Edit: To answer your question: assume the box is moving with some constant velocity v. There must be a net force to change it. That’s it. Force can only affect acceleration, or rate of change of velocity.