Because microwaves are mostly used to reheat ready meals, not to cook from scratch. That's potentially a public[1] health problem if there's little regulation of the sugar / fat[1] / salt[1] content of those meals.

[1] Public health talks about the whole population, not just the individual. This means you look at risks across a large group of people. Some people eat fat and are fine, some people eat salt and are fine, but across the population these things are eaten too much and they cause harm.