Felicity, I wonder if you've read the excellent work of numerous scholars on the existence of Thor, Loki, Shiva, Aphrodite, Krishna, Vishnu, Baal, and the numerous other gods that have been presumed to exist during the lifespan of humanity.
There is plenty of evidence for the existence of these gods. There are temples, holy books, millions of worshippers for hundreds, if not thousands of years. All of these point to the existence of these gods. The fact that temples, statues, books, etc. exist, must mean that these gods have and do exist. Do you agree?
For the same reason, there is no historical Jesus, and no, I don't conflate or confuse a 'historical' Jesus with the religious Jesus. There simply is no evidence for his existence.
Do you think that because the Egyptian Book of the Dead exists that Isis, Horus, Seth or Anubis existed?
Take, for instance, the work of the Jewish scholar Josephus. Until the publication of his work in 1000AD, there was no mention of Jesus. Only in the work copies after that does the name of Jesus appear.
Reza Aslan could not possibly provide information about the 'historical Jesus, because there is no historical Jesus. There are very specific meanings in science to the word 'historical' and 'evidence.'
Evidence would include DNA of bones, photographs, videos, etc. Aslan studied theology - he wouldn't know a piece of evidence from a piece of pie.