On the contrary, they’re brilliant design!

What you want to control is water temperature and (separately) water flow rate. Two taps let you control those but only by guessing the right combination of settings with trial and error. On your first try you’ll get one right (the flow rate, say) and mess up the other one (the temperature, say). Try again and you’re a little closer, but not perfect.

One-handled taps align their control vectors with the search space’s basis vectors. You want more flow? Turn this way, exactly to the right setting. You want higher temperature? Turn this other way, orthogonal to the first, without altering what you had set there. There’s no comparison.