Actually they did come down from heaven because they were standing on earth.
In John 3:13 Jesus was saying that no man can ascend to the father to learn God's will and then return to earth to teach mankind. Only the one who descended from the bosom of the father in heaven is able to teach mankind God's will.
Verily, verily, I say unto thee, We speak that we do know, and testify that we have seen; and ye receive not our witness. If I have told you earthly things, and ye believe not, how shall ye believe, if I tell you of heavenly things? And no one has ascended into heaven, except the one descending out of heaven, the son of man, the one being in heaven. John 3:11-13
It was a direct reference to Moses telling Israel to look in their hearts for how to do God's will, rather than thinking they would have to go into heaven to learn it.
For this commandment which I command thee this day, it is not hidden from thee, neither is it far off. It is not in heaven, that thou shouldest say, Who shall go up for us to heaven, and bring it unto us, that we may hear it, and do it? Deuteronomy 30:11-12
But the word is very nigh unto thee, in thy mouth, and in thy heart, that thou mayest do it. Deuteronomy 30:14