They usually don’t disappear completely—their light is just spread so thinly across the frame that it becomes too faint to notice.

In a long exposure, the sensor keeps collecting light the whole time. A stationary subject projects onto the same pixels for the entire exposure, so it builds up strongly and appears sharp. A moving subject is recorded in different pixels as it travels, so its light is distributed over a much larger area. The faster it moves, the more thinly that light is spread, and the less visible it becomes.

If the moving object is only in any one spot briefly, while the background is visible in that spot for the rest of the exposure, the background dominates. The result can be a very faint blur—or effectively nothing visible at all.

That’s why busy streets can look empty in long exposures, and why moving water can turn into a smooth mist while rocks stay sharp. The ND filter doesn’t remove objects; it simply allows a much longer shutter speed, which makes this averaging effect strong enough to see.