Because that's what we tried for a hundred years and its how we ended up with innumerable wildly dangerous products on the market. The amount of research to vet all the products in your daily life would be astronomical and that's even assuming the companies making them are honest about the ingredients. Here's the context of why the FDA was founded:

> By the 1930s, muckraking journalists, consumer protection organizations, and federal regulators began mounting a campaign for stronger regulatory authority by publicizing a list of injurious products that had been ruled permissible under the 1906 law, including radioactive beverages, mascara that could cause blindness, and worthless "cures" for diabetes and tuberculosis. The resulting proposed law did not get through the Congress of the United States for five years, but was rapidly enacted into law following the public outcry over the 1937 Elixir Sulfanilamide tragedy, in which over 100 people died after using a drug formulated with a toxic, untested solvent.

https://en.wikipedia.org/wiki/Food_and_Drug_Administration

A combination of economists, epidemiologists, public health officials, governments, and individuals.

Why other than you? Because you have an impact on society. Your actions affect others.

But this is to do with food I eat. I don't feed it to other people.

Here's a better question, then: what health and safety decisions do I get to make on your behalf. What can I dictate to you that you can't do, or have to do? Can I mandate that you have to run 5 miles every day? It will be good for you to do, and it will impact others by increasing your productivity and lowering the cost of your healthcare on society. Is it reasonable that I should be able to use the threat of violence to induce you to exercise? Because that's all that regulation is: it's an outline of behaviors for which the threat of violence is a legitimate response.

Would you be upset if you ate something every day and didn't understand the risks fully and then developed a disease because of it? What if no one understood the risks aside from the entity that sold it to you? Would you be upset if someone you cared about deeply, say a child, made a mistake of never understanding there was a risk to consuming something, say, baby food, and then developed a life ending disease because of it? Would you feel responsible if you facilitated giving that person you cared about the food you chose to buy and there by aided in ending their life prematurely?

Any of these scenarios should make it obvious there has to be some sort of regulation around these things, as no one individual is an encyclopedia of toxic substances, and we exist in a bazaar of choices.

There could be a compromise, much like there is with alcohol and tobacco, that if you absolutely wanted to buy something toxic, you could do so. However, that wouldn't really necessitate that you couldn't use it to harm someone else.