Former D3 HC (M/W XC) and Associate HC for indoor/outdoor in Northeast for 3 years. Spent 2 as asst, 2 getting Masters degree. Left for some of the reasons mentioned, specifically pay vs. hrs, among others.
One element more unique to D3 that always frustrated me was the potential for an effective coach to be a valuable asset to the bottom line of the University, yet no route for compensation for the above.
Example: our University's "golden number" was ~$17k, which was what admissions had figured out was roughly how much we netted per student/year. I inherited a team of 12 men, 6 women. In my first year we brought in 8 freshman men, 12 freshman women and retained 7 and 10 into year 2. This was entirely through just putting in the hours of recruiting, nothing terribly special.
Some oversimplified math has, in year one, ~$250k additional revenue for the school (+15 more recruits than previous avg x 17k = $255k). Regardless of how much that number actually is, the fact remains that a good D3 coach has the potential of bringing in dozens of students who wouldn't otherwise be there. These students are typically of a higher academic calendar than some of their student-athlete counterparts, typically are better behaved, and you can have one coach recruiting both men and women. Also, the cost per student for the sport is incredibly low compared to other sports (low equipment costs, fewer significant injuries, no training staff at practices/games).
I presented the info above, along with many more figures, to our President, AD and VP of Enrollment Management. They said it was wonderful info and offered me a bump to $49k. I took a job at a local company and got a $10k increase in salary year one and coach at the local HS now, which is also a lot of fun.
I miss being around those athletes, who were great kids, but it was frustrating working my tail off, knowing the value to the school, and making $20k less than our 7-13 basketball coach.