Dr. Jackson is his ascended illusion form
Why do enlightened beings always wear sweaters? Perhaps when someone achieves ascension to higher planes of existence they also upgrade their fashion sense. It makes sense. Sweaters are comfortable and they keep you warm. What about the V-Neck? That usually says that one is not afraid of showing his/her throat chakra. I guess a V-Neck Sweater makes more sense than a leather jacket or trench-coat. Why doesn’t Dr. Daniel Jackson just wear a robe? Maybe he isn’t quite ascended enough to sport a robe. Or maybe since so many of the villains in Stargate SG-1 wore hooded robes like Sokar and Anubis, it would make sense for a good guy to wear something different.
