As I mentioned in a sibling comment, 68% of US people are Mg deficient, 3% are K deficient. And considering how many are hypertensive, K supplementation, which is already tricky, needs an even bigger caveat, as many hypertensive medications are potassium sparing, thus supplementation should _not_ be done without medical supervision. I agree that it's tricky to test, and the science around potassium metabolism with HTN medication is confusing and contradicting: you can find studies for the same medication that say it's potassium sparing and another that says that hypokalemia is a common side effect.

You can have vegetables containing potassium, sure, but it's a bit of a stretch to say that potassium deficiency is widespread, in general and supplementation of it is discouraged.