I've asked myself this too, and it occurred to me that there are a fair number of people on HN who are "older" (even older than me!) and Defense/Aerospace were pretty common places for techies to work in, say, the 80's. Often the only game in town.

That, and I think tons of tech-oriented people love flying machines, and a good exit or a fat Google salary makes it much more realistic to exercise that passion in real life.