This is because, although the body might exist in three dimensions, or two, or even more, it can't possibly do anything that needs more than one dimension to describe. This is when you let the only dimension be the forwards-backwards dimension through which the body is moving. As such, it is referred to as "one-dimensional" motion because no extra dimensions are needed to figure anything out about the body through problem-solving, provided that, as you say, it just maintains a constant velocity forever.
You also assume something that is incorrect.
It can still change its direction and have constant velocity.
If it were to change its direction, it would have changed its velocity. It would have the same speed, because it is moving through the same distance every unit time, but not the same velocity, because the way it is changing its position has changed. As such, changing its direction of motion is not an example of constant velocity, so the principle is upheld, even though you do often require additional dimensions to describe motion that changes direction.