Good point.
I was originally a PhD student (left w/ an MS). The reason I left and didn’t try to go part-time is because (1) I obviously would’ve lost funding and (2) I know my advisor would’ve lost any interest in me.
Now, Since I’m working a full time job, they cannot fund me as an RA/TA. So I’d pay out of my own pocket, or the company would pay. Wouldn’t it be appealing for them to have a self-funded student? Why are part-time PhDs less common at high level universities?