I couldn't let my advisor down. After sticking with me for two years, I would have felt horrible if I had left him after my masters. I did not _expect_ this going in, and I planned to have "leaving after my masters" as a real option.
To be clear, I still would have left if I had had a good reason. Ultimately, I wanted to stay. But if it had been borderline, I would have still stayed. And "borderline" for grad school may very well be "I'm tired of killing myself with overwork, I don't really feel like doing this shit for a few more years. But I could just suck it up and keep going, since I'm already like half way there." Grad school can be pretty crappy.
I definitely could have turned summer internships during grad school into full-time work, though, and gotten a well-paying engineering job.