Aside from the well-known toxicity of potato stalks, leaves, and flowers, and of the tuber once it has been exposed to enough sunlight, even the edible tuber is usually poisonous enough to noticeably irritate your mouth and throat if you eat a mouthful or two raw. You have to cook it to make it edible.

There's a couple of orders of magnitude of errors in your comment: it's hundreds of millions, for centuries, or tens of millions, for millennia.

But potato has nothing on cassava, which has also been a staple for tens of millions for millennia. Looks like you're a Yank, like half the people on this godforsaken website, so you might only know it as "tapioca". In both cases, being poisonous discourages insects and other wildlife from expropriating your crops, and James C. Scott famously argues that being underground makes it more difficult for a certain kind of human pest to expropriate them as well.

So the humans deliberately eat roots because they are full of cyanide. Truly they are space orcs.