No, that's not how insurance works, but it's how they want it to work. Usually when you get insurance through e.g. work, you get put into a pool that averages the costs and risks across the pool. That makes it's more affordable and is why "insurance for a single person" e.g. for sole practitioners is not a thing.

Also, usually when insurance companies flag you for things like pre-existing conditions they don't just make it more expensive, they just deny it. Or if they do allow it it's so expensive so that it doesn't make sense to buy.

In other words, if insurance worked the way you think it should, there would be no point to it at all, you may as well just pay your costs out of pocket. Personally, I think that idea is 3rd world country level shit. Health care should not be for profit.

> If my diet is the cause of a medical issue, then I'm the one at fault.

This makes sense for drugs with strong correlations to specific medical issues. Alcohol usage -> liver failure. Smoking cigarettes -> lung cancer. Trying to generalize this to entire categories of food is farcical, because most food isn't bad by itself unless you eat too much of it without eating other things. And if insurance companies are given the power to set premiums based on diet, they certainly aren't going to burn money and effort trying to tease apart the nutritional nuances of every possible thing you could eat. They'll instead be incentivized to do the "easy" thing and impose blanket dietary restrictions under their insurance plans, and use any violation as an excuse to deny claims or increase premiums.