Because modern day militaries are a completely different beast.
Far less prestigious, less science and exploration etc, deeper moral ambiguity etc.
Nevertheless the military is the way to get into a lot of exploration, through eg spaceflight or getting posted to Antarctica. Personally I think people think Picard wouldn't join the military because they wouldn't join the military (neither would I!), and they want him to be just like them. We saw in Tapestry how he would have stuck with the military life even without the exciting exploration, etc.