It is entirely plausible he exists...the stories were often told to people so they could relate to him, but I am relatively certain he is a real person.
First, examine the characteristics of "prophethood" in that region. The people there were living in a semi-arid climate keenly aware of the tales of great empires and kingdoms which came and went, and now lay in ruins buried under the sand....
Many of the deserts were once fertile valleys...but exploitation, war, and climate completely drastically changed the landscape overtime, and each time an empire fell people were scattered here and there- some keeping the name of their fallen kings, some returning to nomadic lifestyles conglomerating with other tribes...
Anyways- in all of this historical flux and chaos there were stories about why the empires fell (many had to do with punishment from god for not heeding the warnings of his prophet, or a promise to rebuild itself with the help of a messiah). So this was already pretty much a part of the local folklore no matter what religion or race you were. (perhaps with the exception of the foreigners from the western empires- rome and greece)
Well, what does a prophet bring? Pretty much every influential and well respected prophet has been a person- a human- who basically figured out that society is bullshit, that people are unnecessarily cruel, and that we need to change the way we think about living and about each other or else we will destroy ourselves with greed and fear. (like what we are doing currently with over-exploitation of land, animals, and peoples)....
Thats why prophets always come at a time that is particularly chaotic. They come with a vision, and ignite a movement helping the people free themselves from the clutches of a corrupt society and group-think, and encourage individuals to seek what is just and true, and stand up for it. In order to do this people need to be free, and they need to be encouraged to think/philosophise...they also need to be taught to respect one another and live in a way that does not step on each other's freedoms... so the prophets tend to speak in parables, or poems, or something to provoke critical thought.
Well, what usually happens is, the prophet is really well respected by some people and the people are motivated to carry the message to others, and as they do so (being in an area where various cultures interact), they have to be able to relate the message in different languages, cultures, etc. And because people are so inspired by what the prophet does, they romanticise them to a great degree...you can see it in the way people romanticise their favourite leaders.
Overtime the stories become embellished, etc and then ta-da you have what sounds like a nice story and doesn't seem very relevant. Whats more, the people spread out the message lose the whole point- the point is to think, reflect, do what is righteous...but instead they get sucked right back into group-think, follow-the-herd mentality...
The reason prophets can keep emerging is because societies keep screwing up and people keep forgetting to think. So when someone actually manages to think AND speak about it, the people assume they are a prophet.
