That's what I always hear, but when you see someone sneeze or get that rash from that thing, you aren't seeing the thing, you aren't seeing the allergy, you are seeing the effects of that allergy: the allergic reaction. Certainly, you cannot see anxiety or allergies like you can see that someone is missing an arm or leg, or that someone is using a cane to help them walk with their blindness or troublesome back, leg, hips, what-have-they. But you can still see the effects of anxiety just like you can with allergies, you just have to look harder.
People will spectate anything. Whatever is happening, if there isn't something else going on that grabs their interest tighter, they will watch anything. What people tend to not do, however, is watch closely and with good intentions. People are selfish, people won't work to watch things, even if there is a good reason to do so. And that's the issue with "seeing" something like anxiety, it isn't something people want to watch, especially not closely. It's something they don't care to do and it's something that requires effort on their part.
If one would watch and listen closely, I think after you get past that first hill and look over it and into the valley that the effects of anxiety, of any mental illness and other things, are easily noticeable: "sleeping" in late (laying in bed, eyes closed, feeling miserable with anxiety, depression, or even paralyzed by them), cancelling plans, appearing exhausted in any form (tired eyes, slack face muscles, dreadfully deep sighs, slogging steps), or even not doing something or not being visible or loud when one used to be, being silent for a day, week, month, season...
People won't believe mental illness is visible, yet they will believe allergies are. I find that those people tend not to have their minds open, or even have them barred shut to such things like anxiety, OCD, depression, insomnia. Stress. Even with themselves, people will bar their metaphorical doors to the idea of stress, even when they are capable of or actively feeling the effects of it, mentally and physically.
"Yeah, but you're used to it..." - a former partner, talking about insomnia with me
I don't think I'll ever be "used to" my anxiety or OCD. What I am used to is how people will misunderstand or fail to see things. I don't really blame them for it, most of the time anyway. My parents, teachers, partners, family, friends, coworkers, sometimes they can't see it, they simply can't wrap their mind around it, other times they have their doors barred against it and things like it and are averted to the thought of opening them.
Usually they try to make their way up the hill. Or, I should say, the ones who really care will try, sometimes they reach the top with a little help or a little time and look out and see and understand better. Sometimes they don't, or just can't. I'm pretty used to that, enough that I don't do much besides sigh and be internally disappointed for a day then brush it aside and move on.
I do wish more people would try, though, and honestly try to understand. Not everything, especially never all at once, but a bite of the pie, less than a slice and more than a nibble. I don't think it's too much to ask most people to choke down a bite-sized piece of understanding every now and then, to open their mind just a crack.