Assumimg that being overweight is due to fat, not muscle.
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A lot of good answers for biological reasoning here but the answer is also in the statistics.
It’s more complicated than just BMI as you allude to and there are numerous measures for unhealthy weight but the ELI5 is that ‘having elevated weight increases your risk of disease/impairment and death’ (morbidity and mortality) which is verified in countless studies (I could link some but they are really very easy to find). Even if we were biologically clueless as to why (we aren’t), this would remain an answer.
“Routine” blood tests are varied and don’t necessarily pick up on all risks conferred from being overweight. Everything in medicine ultimately comes down to risk / probability and trying to reduce risk. Yes, there is elevated risk for bad cholesterol/diabetes which would usually be picked up on a “routine” blood test but even if that’s normal the risk still remains as these tests aren’t perfect markers for all possible ‘badnesses’ in the body some of which remain beyond our complete understanding.
Because person A is overweight and person B is of a normal weight doesn’t necessarily mean person A won’t live a longer life than person B but we would say that person A is at higher risk of earlier morbidity and mortality than person B (regardless of what routine blood tests show). If you have bad cholesterol and/or diabetes that just additionally increases risk.
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