because not all magic energy is the same, if you were a water magic user and absorbed the fire curse for example, it would be as if water magic is clashing with fire magic because your entire body IS fire magic, and it would cause you immense pain to try to use water magic afterwards. It works the same way with other magic types, because the curse is a specific type of energy it will reject any other type of energy
Said by vetex directly at How exactly do curses work? - #37 by anon1881879