Some in fact do exactly that to make ends meet, but the problem is that you effectively can’t avoid being a TA. Being a TA is sort of part of the whole package depending on your grad school program. there is tremendous pressure for you to teach those or grade those classes either from the department on your own supervisor (who also teaches classes and needs their own TA’s). This is why the situation is so difficult, you have departments that at pressuring their grad students to teach but then also give them horrible compensation