Why do people find it concerning if someone's BMI is not in the healthy range?

I heard that BMI is not a very good way of measuring someone's health, like if someones BMI is underweight then people would be concerned and think they have an eating disorder but then the NHS tool for BMI says not to use it if you are getting help for an eating disorder and everyone automatically thinks that person has an eating disorder, whilst an overweight person probably wouldn't get picked on as much but people are so damn obsessed with BMI. It's not just BMI, it's genetics, ethnic background, the family's heights and weights/family history and look into their growth and loads of other things that can affect a person's weight as well.

BMI isn’t as accurate as other indicators like those using waist circumference to height or hip circumference. It doesn’t distinguish between muscle, fat, and bone mass, however, a BMI above 30 (obese) has been proven a countless number of times scientifically to increase the risks of debilitating conditions.

As an example, I’ve published research on increased risk of degenerative bone conditions like osteoarthritis and osteoporosis. These are mostly caused by reduced range of motion and lower activity level associated with being clinically obese.

It’s not always of interest academically how someone became obese, the increased health risks (or the hypothesis of the study) are apparent regardless.

In fact, if you look at the UK average BMI of 10 year age brackets, data shows the average BMI of people starts to drop after 70. They’re not getting fitter, you have a reduced life expectancy if you’re obese.

I didn't know that osteoporosis was linked to reduced motion in activity linked with being obese, I actually thought it happens only if you are malnourished or anorexic and your bones are weak and can crack easily that's what osteoporosis is. During anorexia treatment, therapists may check the health of your bones and even my geneticist would have checked my bone age as it's linked with my genetic condition but there is no point as I am an adult now therefore I am not going to grow anymore and my bones are fused. I have a growth disorder causing digestive issues.

When your doctor tells you that your bones have stopped growing, they merely mean that your growth plates (typically located just before joints) have calcified (hardened). They’re still very much alive and strengthen and/or degrade as to efficiently match how they’re used for the rest of your life.

I didn't know that osteoporosis was linked to reduced motion in activity linked with being obese, I actually thought it happens only if you are malnourished or anorexic and your bones are weak and can crack easily that's what osteoporosis is. During anorexia treatment, therapists may check the health of your bones and even my geneticist would have checked my bone age as it's linked with my genetic condition but there is no point as I am an adult now therefore I am not going to grow anymore and my bones are fused. I have a growth disorder causing digestive issues.

I read this and felt like i could relate. My bmi js significantly under then normal when they calculate it and i was constantly bombarded by "oh gosh, that's really low ' you're really underweight, you look really skinny and made automatic assumptions towards being anorexic etc... This even included health professionals, relatives etc... Everyone! Until i started build a relationship with my GP and she now doesn't make to much of a fuss and says yes you're under but you're functioning and your not actually that significantly under an bmi like shows, she is against these bmi calculations etc and doesn't rely on it heavily. I mean sparked up conversation regarding food and yes i have autism which makes food a challenge and a muscle condition which im uncertain if it's effects my weight etc... Now i don't let anyone else weigh me or anything other then her.
But yeah i was fed up with that automatically assumption and i still got these comments etc but at least there's one person who gets me.

I hate these comments and I am surprised I don't have body dysmorphia or gender dysphoria but instead I have an eating disorder called anorexia nervosa. My sister gets the complete opposite with people fat shaming her even when she was of a decent weight and now she looks back at past pictures and thinks she looked so good before compared to now as she's overweight. I am always skinny shamed or being called anorexic whilst she's always called fat. I think people's comments about my weight has made me develop an eating disorder to be honest and even though people call me skinny or anorexic (and that's supposed to be a good thing rather than fat shaming and being told to lose weight and that's the way society sees it) I still don't see it as skinny enough from my view and it's my perception of things, I look down at my body and I see it's normal but in the mirror it looks completely different and thin, people don't see what I see.

I hate these comments and I am surprised I don't have body dysmorphia or gender dysphoria but instead I have an eating disorder called anorexia nervosa. My sister gets the complete opposite with people fat shaming her even when she was of a decent weight and now she looks back at past pictures and thinks she looked so good before compared to now as she's overweight. I am always skinny shamed or being called anorexic whilst she's always called fat. I think people's comments about my weight has made me develop an eating disorder to be honest and even though people call me skinny or anorexic (and that's supposed to be a good thing rather than fat shaming and being told to lose weight and that's the way society sees it) I still don't see it as skinny enough from my view and it's my perception of things, I look down at my body and I see it's normal but in the mirror it looks completely different and thin, people don't see what I see.

I know what you mean. The comments can really hurt and be more harmful than good : your very much right it can lead to developing an eating disorder. I know i certainly became more aware.
Yeah like being" skinny " people say is a gift and it is in a way better than ttgr other way round but when people make such comments it doesnt feel like it. Are you getting any support?