Blur is geometry plus time. If the part moves more than the smallest feature you care about while the shutter is open, that feature is already averaged across neighboring pixels.
Comparing a few milliseconds of exposure with tens of milliseconds on a moving resolution target makes the effect obvious. Gain is not a substitute. A strong digital gain stretches both the signal and the dark noise and leaves gaps in the gray scale. The image looks brighter and grainier. Aperture trades the same three things: a small opening buys depth of field and costs light and granularity; a large opening does the reverse. Adding light is usually the clean fix because it brightens the image without forcing gain or a long exposure. Partial acquisition and trigger delay belong in the same conversation, because they decide which rows and which moment become the image.