Cost presumably. Those thrusters use hypergolic propellants which are corrosive so would need to replaced after every test. Now that's not to say they couldn't

Cost presumably. Those thrusters use hypergolic propellants which are corrosive so would need to replaced after every test. Now that's not to say they couldn't have done it - the LEM from the Apollo days also used hypergolics and they did test those on the ground.