Why are teachers unions against merit pay?

Merit pay is a joke unless U R telling from a technical field. Who would judge and how would U judge who is a good teacher and who isn't except for the really bad ones, U really would take a long time to find out. All of my kindergarten kids did well, have had self esteem, very productive citizens. Who knew? I haven't taught for years. Go get my money!! Buddies would nominate their friends, not necessarily the better teacher.