Read: Joseph Campbell's works. He has several books out and is considered the scholar having to do with Myth and Relgion. The book "The Hero with 1000 Faces" is quite on point for this discussion.
Here let me help you.
Sorry, I would give you more links but they are blocked at the work computer.
http://www.kirjasto.sci.fi/campb.htm Or rent Power of Myth where Bill Moyers interviews Joseph Campbell for several hours.
I googled your question " jesus reworking of other myths " and got quite a few hits. Instead of asking why not research it yourself or is your study of Greco/Roman, Celtic, Buddhist, Egyptian and other civilzations that rusty. They all have deities that sacrificed themselves for mankind (Prometheus to name one). They all had death and resurrection themes, they all had savior motifs and many had huge floods, plagues and other such godly things.
Sheesh....
Oh and not to put too fine a point on it, the one thing that both pagan and mainstream religions have in common is this. Both have writings that those who worshipped those gods followed and none of them have any actual artifact proving beyond a doubt that they are correct. The one thing that the Bible has over say Bastet, is that the Bible was written in more recent times during and as such can reference the Romans and other historical certainties. Does this alone mean that Christianity is more correct? Nope, just more recent.