That is why Paul is a good source. He had access to witnesses. He was in league with the Jerusalem church, which was headed by the brother of Jesus, and two of his leading disciples. More so, Paul and Peter even spent time together, where Paul most likely would have learned about the earthly Jesus. Paul was in a great position. And as far as we know, even though the Jerusalem church had problems from time to time with Paul, they still supported him.
Paul separates Jesus into two figures. There is the earthly Jesus, the one who actually lived, who had a brother, was flesh and bone. This is the pre-resurrection Jesus. Then there is the post-resurrection Jesus. This is the mythical supernatural Jesus you talked about. This is Jesus after he is resurrected, and has undergone a transformation (which Paul speaks about the transformation in 1 Corinthians chapter 15). One has to realize this when reading Paul.
For Paul, the earthly Jesus really didn't matter. It was his death, and resurrection that really made an impact on Paul. It is the post-resurrection Jesus that Paul is concerned about.
There are a few reasons for this. First, his letters were not intended to spread information about Jesus. Paul had already done that when he went to those areas in the first place. Instead, in his letters, he is only addressing problems and questions that had arisen. Those people Paul is addressing already were followers of Jesus, and we can safely assume that they were already informed about Jesus (otherwise they wouldn't have been in the church).
More so, as I said before, Paul was more interested in the risen or post-resurrection Jesus anyway. That is who we do see in the epistles of Paul for the most part (even though we do see from time to time Paul mentioning the earthly Jesus).
Add to that that we only have a small amount of what Paul actual believed, it is no wonder we don't have much about the earthly Jesus from him. We only have a few letters. There are other letters that are now lost, and there would have been all of his preaching that we simply don't have. We can't really judge Paul completely just based off of his letters, as they only give a very small view of what Paul believed.