Just because there are some things the government does not protect people against, does not mean that investments are the only thing the government protects people against.
> State lotto
Most gambling is either highly regulated or outright banned.
Also, gambling is a financial product, which arguably falls under the category of "investment".
> Supplements/vitamins of dubious value
Because these products are very carefully designed and marketed to avoid the heavy regulation that is applied to medical products
> Regular stocks with wildly ridiculous P/E ratios
These are investments, so this example doesn't really fit into your thesis that the government only regulates investments.
> Various forms of "insurance"
Insurance is a financial product that could also be viewed as an "investment". Further, many types of insurance are highly regulated.