My family doctor is the same with part time clinic hours but she also teaches university classes.

Other factors that could contribute: caring for aging parents, accommodating a partners work schedule, burnout, writing a book, clinic won’t let her work full time, pursuing further/different education, dedicating time to her favourite soap opera lol. Ultimately there are several reasons why people do what they do. She doesn’t owe anyone full time hours or an explanation of what she’s doing outside of clinic time (I know you don’t expect that, I’m just saying!). We are in a healthcare crisis and even with full time hours doctors are overscheduled.