Can you generate your own HDRI maps without using photos?
Yes, it is possible to do this. If you use a tool like Photoshop in 32-bit per channel mode, any values less than 0 or greater than 1 should be usable as a high dynamic range values. I don't know that I agree with your contention that "people don't seem to do this". Given that the tools exist, they must be supporting some workflow.