To get a quote, you just need to be able to fill out a form online. To get the insurance, you either need to be in some form of the military(active, reserves, IRR, guard, etc.), have an honorable discharge, have a general discharge under honorable circumstances, be the spouse of someone who is one of those, or be the child of someone who is one of those. There are plenty of people who have USAA insurance that have never served a day in their lives.
6.1% of Americans served in the military, but that number has been dropping. In 1980, it was 18%. That means there are a lot more people who are children of those who served in the military.
From my understanding, USAA is about to expand to grandchildren too. As the veterans die out, and their children die out, they now plan on selling to the grandchildren.