Why is saying someone who’s very overweight is unhealthy considered body shaming ?

Because here in the 21st century it's just about what feels good. Facts are fungible, hurt feelings are the same as physical violence and any limitations put onto anyone, even those inflicted by basic biology, are an "ism" of some kind and justification to destroy the livelihoods of others. 

Obesity and syphilis are similar. Both are unsightly and fatal if untreated. We know what causes them, how to prevent and cure them. Encouraging fat or fatal STDs in any way is wrong. NOT body shaming encourages it.