Because how else are you going to create a martyr myth?
I personally think such a historical figure as Jesus probably did exist- probably some kind of itinerant prophet/faith-healer. He also was probably crucified by the Romans for being a rabble-rouser. They did that commonly. Such prophet figures were not uncommon in 1st century Palestine under Roman rule. One who was probably even more famous than Jesus in his time and a near-contemporary:
en.wikipedia.org
There also was probably a cult that grew around this cult prophet, a heretical Jewish off-shoot. At first, the leaders of this cult really thought of it as purely a Jewish religion. Peter was outraged with Paul for going around trying to bring in non-Jews, and in the process getting rid of the laws and requirements that defined what it meant to be a Jew in the first place. After all, Jesus had said himself that he had not come to subvert the law. But this thing got beyond them and took on a life of its own.
Many centuries later, the emperor Constantine wanted to find a religion to unify his fragmenting and disunified empire. He recognized that religion can be a powerful unifying political force, and just happened to pick Christianity under which to do it. It was a contingent choice. He had other choices. But even the Christians couldn't agree on any unified or coherent set of ideas or beliefs. So Constantine convened the Council of Niceae and gave them a deadline to come up with a coherent religion. It didn't matter to him what it was, as long as it was something. The emperor himself even personally attended this and breathed down their necks until they did it.
The rest, as they say, is history.