Probably a few reasons. One, engineers are generally thought of as "smart" people (as a practicing engineer, I can tell you this is most definitely not true) and in general, people think you need to be extremely intelligent to fly an airplane. Not that you dont need to be smart, but simply flying isn't rocket science....unless your an astronaut, then it literally is. Engineers are also problem solvers by trade, which bodes well to flying aircraft in general and in an military environment I would guess as well. Not to toot my own horn, but acquiring and engineering degree not easy. I would say it is difficult for 90% of people that get them. There is a lot of complex information to learn while getting that degree and I think that bodes well to the mindset you need to have going into pilot training. Bottom line, the degree you have doesn't mean you can fly a plane better than the guy next to you, but I think it does influence your train of thought, problem solving methodology, and critical thinking skills. All that said, you still need a good GPA. That is what is important. Cool, you got a degree in particle physics? Well that doesnt mean diddly if you graduated with a 2.5. A 4.0 in basket weaving is better than a low GPA in engineering. To add, I know a A-10 pilot and a F-15C pilot, both have degrees in Crop/Soil science or something like that. So it really doesn't matter.