My information is dated for sure, so I really can't speak for those waiting for pilot training. I'm sure it depends on the wait, some will be assigned to a job at their UPT base. I had friends that were in "casual status" while waiting, and that was just doing small jobs until their UPT class started.
My interpretation of why pilots might not go to grad school? Once you graduate UPT, you will go to your follow on training. Lots of options here, so I won't get into that. But once you get qualified on your new aircraft, there are so many quals left depending on your type of aircraft. It can take many years to achieve the highest qualification level, and most pilots want to achieve these levels. Most think your company grade years (O-1 to O-3) are about becoming the best in your weapon system. Field grade years are time for follow on schools (professional military education) and leadership positions. Most pilots don't want to leave the cockpit to go to graduate school, and honestly, most aren't afforded that opportunity. Most are expected to fly and be operationally proficient.
Masters degrees are important to make field grade. I'm not sure of the latest, but at one time, this information was not supposed to be a part of your promotion record. But, it's there for sure. Commanders know and they will assign a ranking for each Officer meeting a promotion board. So many of the pilots I know (including myself) took masters classes at a university that had office space on base. For example, Embry Riddle offers masters degree programs at most larger bases.
Many pilots seek to go to airlines after their initial commitment. They just want to concentrate on getting as many flight hours and qualifications as possible to improve their chances. I can't think of an airline that really cares where you got your undergraduate degree, much less if you have a masters.
When officers get a chance at going to a graduate school, they have an obligation to serve extra years for this benefit. But, almost all of the time, this time runs concurrent with your previous commitment. Effectively, it adds little to no time to your commitment. It really depends on how much commitment you have left. For example, UPT has a 10 year commitment AFTER pilot training.