Can you explain why you believe that, of all professions, it is impossible to assess performance of teachers?
Several comments in this thread detailing why the teaching profession is a different dynamic. Either you haven't read this thread, or dismissed those reasons as irrelevant.
EDIT: To add to the other comments. Here's a reason you perhaps haven't thought of: Great teaching, not just good, is about making an emotional connection with a kid. Think back to your great teachers. The ones that had some form of impact on your life. You probably don't choose that teacher because of the facts they helped you learn. They made you think more of yourself. They made you think you had potential. Building on this idea, which is the better teacher? They one that guides you to a wealthy life, or the one that guides you to simply being a happier person? Which do we value? Almost 100% of parents would choose to have their kid have a happier life as opposed to a better test score. How do you objectively measure that? I'm all for you trying, but good luck finding a way to pay me more. Again, this is about paying me more for being successful, right?