Could it be because HFCS, like Sucrose is about half Fructose and that Fructose is only metabolized by the liver... Not to mention the damage Fructose causes, similar to glycation but without anything resembling reliable testing. So basically everything bad about alcohol for the body but without the negative reinforcement of a headache after.
Similar for fruit juice. In nature, people weren't drinking fruit smoothies by the liter year round either.
When I was a kid, my grandmothers had these tiny 1/2 cup glasses for "fruit juice" which was a serving.. and you should "only have one serving a couple times a week." That's just not what people do today.