Because the last 50 times he did it, people thanked him for his service and didn't bother to ask questions. No decent person would lie about something like that so it's considered inappropriate to grill them about it. I'd imagine asking every service person "did you really serve?" will get you more fights than answers because its kinda fucked up to assume someone is lying about it, just as much as its fucked up to lie about it.

Then he did it the 51st time here, thinking she would just go "wow thank you for your service" and move on. Turns out he happened to do it at a real marine, and here we are.

Edit: to clarify, I'm not trying to say people usually don't talk shop when service members meet each other. I'm saying that this dude basically went years saying he was a marine to average Joe's, who wouldn't have any questions or way of knowing if he was lying or not. I assume it's super rude when someone mentions their service to ask them to "prove it" so this dude ran around lying for years. This lady was the first marine he did this to and I can tell because his face appears to have not been caved in (yet) by a real service member for lying and commiting stolen valor.