Oh, there probably are 500 retired generals and admirals endorsing Romney........
And every single one of them is double dipping with a defense contractor or some beltway bandit and collecting six figures for "consulting", on top of the fat pension and benefits they already get.
Naturally they'd endorse Romney.
Romney promised to fatten the goose that's still laying golden eggs for them!
There are lots of reasons I'd expect more retired generals to support Romney than the general population. For starters, they're a vastly disproportionately male group, and males are more likely to vote Romney. They're also disproportionately white, and white people favor him. They're also wealthy (even those who aren't at the defense contractor trough earned the equivalent of six figures for years, and then retired with excellent benefits), and wealthy people favor him. They're probably also disproportionately southern, since the south has tended to have more of a militarized culture, and that also favors Romney. I'd even wager they're less likely than the general populace to have friends and family who are immigrants, and thus Obama's stance on immigration reform isn't going to be a big draw.
Plus, much of what Obama offers is stuff they get anyway, so there's not much for them in it. For example, how much use is a jobs program when you're already retired and living off a very comfortable taxpayer-supplied income? How much use is Obamacare when you already have socialized medicine that's far to the left of anything Obama is offering? How much use is student loan forgiveness when the government covered the cost of your education already and when your kids are much more likely to be going into ROTC or the military academies?
Then there's the fact that these men are specialists in war, which will naturally make them quicker to reach for it as a tool. That's just human nature. If you go to a doctor who specializes in a particular procedure, he's going to be more likely to see that as the appropriate procedure for your condition, even if an impartial doctor would suggest some other course. For example, surgeons are quicker to recommend cutting someone to deal with heart disease. It's just human nature to be biased in favor of thinking that the best solution for a problem is the one that calls on your own expertise. So, generals will, naturally, be biased towards military solutions, and right now it looks like Romney will be biased more in that way, too.
Military personnel are also a self-selecting group that values authority, discipline, tradition, and directed violence more than the general populace does. That fits in with the Republican mindset better than the Democratic one. These are, to a man, people who chose to spend their careers following orders almost absolutely, wearing uniforms, and killing strangers for money or glory or love of country. That's a blunt and probably offensive way to put it, but it's an accurate description. I don't mean it as an insult. Our society needs that type to defend it, because other countries have that type to attack us. But, in every country, I'd expect there to be a tendency for career military volunteers to have a conservative bent.
So, there are LOTS of reasons I'd expect more retired generals to endorse Romney than Obama (notwithstanding some big counter-examples like Colin Powell), even aside from the fact a large portion of them have a vested financial interest in favoring whichever politician proposes the fatter "defense" budget and the more interventionist foreign policy.