If empathy is supposed to allow one to ‘feel’ someone else’s emotions; why is it that no one can seem to tell when I’m upset?

And I’m not talking about when I mask and hide my feelings by trying to act the same until I can get home and crash. I’m talking about when my entire demeanor suddenly changes: my body gets rigid, you start making odd gestures/movements (stimming), and your eyes well up with tears, and you get real quiet (I’m sure it’s different for everyone, but you know the level of upset I’m talking about, when you’re on the verge of a meltdown).

Even when you would think your body language would be a dead give away to how bad a situation you’re in, no one bothers to ask you what’s wrong or what’s going on, no one tries to comfort you or defend you, it’s like they don’t even notice. Either that or they don’t care, I don’t know which is it.

Why is it, when, even as someone who is supposed to struggle with empathy because of my autism; does it seem like no one else has any, like the whole idea is a myth/rare trait in people?

I know people aren’t mind readers, but still; I’d think when someone is that upset people would notice and want to help; instead it’s not until you finally break down and cry that they try to offer help. Though, given all my observations, it seems they only do it at that point to get you to shut up because no one wants to hear your crying or see your meltdowns, because it makes people uncomfortable and it’s imperative to them to maintain that social equilibrium at all times regardless of the costs to individuals. Weather they actively realize it or not.