In ancient times it was not uncommon for high priests to position themselves as the mediators between God and the people, and anyone wishing to gain God's favor had to appease the priests to get it. This was pure fraud.
The gate-keeper scam, in one form or another, is still around, and is actually so common that we have come to consider it to be normal religion, and we don't think of it as fraud per se. But if we observe the same sort of behavior in a different context, we easily recognize it for what it is.
Suppose that we were talking not about prophets, but about tour guides at the Grand Canyon. If they claim to have dug the canyon themselves, we should be suspicious. If they say that their daddy dug it, we should be almost as suspicious. If they insist on going to the canyon and taking pictures that we can buy, and not allowing us to see the canyon ourselves, we should ask for our money back. The job of a tour guide is to make the canyon accessible to us, and to deepen our appreciation of God's work. If the guide gets in the way, we should complain.
Similarly, priests and prophets are not the main event — they're just other spectators who might help explain what's going on. Some are more observant than others, and some are more eloquent. But this does not mean that they have metaphysical powers — they're just talented. People who can make God more accessible to us, and deepen our appreciation of Him, are prophets. But if they call attention to themselves, we should politely ask them to step aside. God is the main event.
After all, at the end of the tour, do we want to remember being brought into closer contact with God? Or do we want to remember listening to a great tour guide? Put another way, if there was a canyon without a tour guide, we would still go there to see it. But if there was a tour guide standing in the middle of the desert, hundreds of miles from the nearest canyon, we wouldn't stop and pay the fee to find out what the tour guide had to say.
Most of all, anyone claiming to be descended directly from God is to be scrutinized carefully. God is everywhere, and in everything. Therefore, He is not present in one person more than in any other, and no one is special in this respect. We naturally personify God in order to understand Him better, but unscrupulous people can twist this into a scam. If they can get the personifications associated with real people (i.e., themselves), we'll worship those people instead of God. Hence deifying a real person would otherwise be impossibly presumptuous, but if we have already personified God, it's just simpler to think that He is a specific person, so it's easy to do. But it's easy to spot a false prophet pretending to be God. Invariably, if He became human in order to teach us how to live, we can expect His lifestyle to be exemplary. So imagine a world in which everyone follows that example, and lives exactly as that person does. Would that be a better world, or would that actually be worse? Therein lies the difference between a prophet and a fraud.