Why do my stove elements heat on high regardless of the knob setting?
If the elements come on and get hot, they are fine.  There is no circuitry or "smarts" to the element.  It's basically just a thick wire that gets hot.

The knobs in the stove connect to controls that set the amount of current that flows through the elements.  If they only go on high and can't be adjusted there is a problem inside the stove that's probably beyond the scope of this answer (so many different types of stoves and several things that can go wrong with each).