Why do Muslims say “Peace be upon him (PBUH)” after saying the name of a prophet of Islam?

It's said in respect. Prophets were the men who has taught humanity the meaning and purpose of life. So we respect them all. It is not that they require our prayers, Instead it is we who are rewarded from Gold Almighty for sending blessings upon them. Prophet Muhammad (pbuh) did not teach us to say this to him alone, he said like you sent blessings on me, say it for Abraham, Moses, Jesus and all other Prophets of God Almighty. So Muslims keep it a practice to say this whenever the name of any Prophet is heard -Peace be upon them all.