I heard Boeheim talk about this once, at one of the luncheons he used to host weekly.
Paraphrasing, obviously -- but his take on it was that once he got the head coaching job, he was making quite a bit of money. Not compared to what coaches are paid now, but relatively speaking. And the cost of living in CNY is lower than some other places, and he wasn't into flashy cars, etc. -- he's driven a pontiac his entire adult life. And he wasn't into flashy clothes -- in fact, he preferred wearing sweats / SU gear, most of which was supplied by the university.
So he was making pretty good money, and wasn't spending a ton / living extravagantly. And over time, it added up. His house was paid off, etc. And that just enabled him to save more.
And then in the early 80s, he got the 7 figure personal contract with Nike. He allocated some of that to assistants, but it just contributed him to having more money than he knew what to do with.
So later in his career, he just didn't see the need to chase dollars like many other coaches. He was where he wanted to be, doing what he wanted to do, and getting paid insanely well for it. Was he earning what his market value was? Probably not -- but he didn't feel compelled to squeeze the university for the maximum, because he already felt like he was well off. What are a few more bucks, when you've already got more than you can possibly spend?
He also didn't begrudge other coaches for doing so, and trying to get maximum $$$. Just that for him, he didn't need to do it that way.