not having an account. privacy concerns. not liking the rating system. not wanting to skew recommendations. indifference to ratings. or maybe they will rate later or have done so at an earlier time.
stuff like that.
even if you set your privacy options for your followers to not see your ratings, some unprofessional developers have screenshotted those. and of course, if you do have followers and public ratings, you might not want to tell those followers each game you visit.