In Islam culture, people dont like the idea of attaching any physical attributes to Muhammed. Mostly becasue we dont want the religius figures turning into laughing stock, like the western society turned Jesus into comercial products. And again, thx to western monopoly we all see Jesus as a white, skinny man with long hair.
In the Islam philosopy, Quran is the center of religion. Leting people using the image of Muhammed can lead people to treat him like pagan gods. Carrying images of, statutes, toys, toast makers of Muhammed like the western society did to Jesus.
Even now, some people with their weak willpower are giving credits to Islamic saints more value than the actual God.
So we dont want image of Muhammed end up like Mother Marry.