People have weird opinions about what other people should weigh and look like. Maybe the most straightforward viewpoint is that they're used to seeing you a certain way, and it's difficult to imagine you in a different "category."
Some people are naturally very thin for their height, but I think it's more to do with appetite/diet and exercise patterns than anything else. Yes, it's a biological difference, but the difference is what their body makes them do, not how their body actually processes things.
I think you have the ability to be just as healthy as anyone else at your target body composition/weight. And you've chosen a weight in an average range, so you're not being unreasonable.
Try not to listen to the people sticking their noses in your business.