I’d like to see your evidence showing that neither smoking nor being overweight causes health problems for the vast majority of people.
I do not care about statistics here.
Clearly.
Because it is useless when it comes to an individual under huge variety. Humans are not robots made in a factory under equal conditions using a single blueprint. We are a product of evolution and one of the key elements of evolution is production of varieties. Certain body mass ratio might be problematic to one specimen, while completely harmless to another. The smoke can case cancer to some specimen, while others’ immune system is strong enough to kill all the tumour cells. Some people get fat despite doing lots of exercise and not eating sugar/fat, while others stay skinny without exercise even though they eat lots of sugar/fat.