why does my subaru overheat without a thermostat

your car overheats without a thermostat because the water is continuously circulating. when a working thermostat closes, it allows the water to stay in the radiator long enough to cool down. when the temperature in the engine gets hot, the thermostat opens and allows the cooler water in the radiator into the motor and the warmer water from the engine goes into the radiator to be cooled.

That's not how thermostats work. Too much water flow only slows down proper warmup. The water doesn't need to sit in the radiator to cool sufficiently unless there is something wrong with the radiator. Your issue is likely somewhere in the radiator. Are the fans working properly? Is there a blockage, like leaves our other debris, preventing proper airflow through the radiator fins? If all of that is ok then your problem is either in the radiator, something preventing sufficient water flow to cool off the engine, or elsewhere in the cooling system causing the same issue. If nothing is reducing the airflow then get the cooling system flushed and replace that thermostat with a correctly rated one.

It's likely that the overheating is due to a blown head gasket.