Prohibitions on depicting Muhammad arose as a reaction to perceived idolatry; other religious traditions in the Near East were accused of worshiping graven images of their gods, and an Islamic taboo was developed to prevent such practices from creeping in to the religion and to underline the distinction between monotheistic and pre-Islamic practices.

Since there was no such custom related to naming people after gods or prophets, there never arose a taboo against the practice.

Another question: Why is Mohammedan considered an outdated/offensive appellation for a Muslim? I mean they are followers of Mohammed, just as a Christian is a follower of Christ.