The usual explanation of spiral arms involves density waves. Nonuniform motion leads to matter alternately bunching up (boosting star formation) and stretching out. Like cars on a congested road, stars move through regions of greater or lesser density over time. The rotational smearing you anticipate is known as the "winding problem." This Scientific American article offers three explanations by professional astronomers.

The usual explanation of spiral arms involves density waves. Nonuniform motion leads to matter alternately bunching up (boosting star formation) and spreading out. Like cars on a congested road, stars move through regions of greater or lesser density over time. The rotational smearing you anticipate is known as the "winding problem." This Scientific American article offers three explanations by professional astronomers.