Generally, hardware vendors who believe that they have significant hardware knowledge secrets that their competitors do not, will not release source code for their drivers, on the assumption that having access to the source code (which has comments, definitions, names, and so forth) will let someone analyze how the hardware “secret sauce” works.
Presumably, they are also somewhat concerned that someone who just has the source, but not access to the engineers who built the hardware, the schematics of the hardware, and the internal hardware documentation, might actually damage hardware by using it wrong. The support nightmare of that would potentially be pretty costly for a hardware vendor.
If you want driver sources, you can always try looking at the noveau driver, which is an open source driver for NVIDIA graphics hardware, based, I think, on reverse engineering? I don’t know how close or far away that driver would be from working on the Jetson.