It's estimated that 200,000 Americans a year suffer from an iron overload which can cause all sorts of problems including damaging your heart. So it's not considered to be a good thing for a "one a day" supplement to habitually contain iron.
Many Americans are deficient in potassium and you might think, "cap up some KCl and take one a day". KCl can slow down your heart rate and reduce your blood pressure which is good to some extent but if you could buy KCl supplement that gave 100% of your daily potassium need some people would be dropping dead from an overdose of potassium (a potassium is part of the usual lethal injection cocktail), not necessarily because 1 pill is dangerous but because some people are going to take 10. Also some people take pharmaceuticals that either raise or lower potassium levels so what is too small of a dose for some people would be dangerous for others.
The 12,000% of your daily dose for most things is a scam with the possible exception of vitamin B₁₂ for which many people have bio-availability problems.