Why would a car get hot if you have checked all hoses and the radiator?

Check the fan. If you have an electric fan, verify it comes on when the car is at operating temp. If you have a 'clutch' fan you can verify it's working by starting the car and listening for the fan to change pitch or 'roar' when it warms up. This can be quite noticeable on larger fans (NEVER put your hands anywhere near a moving fan. Keep a safe distance). With the engine off check the fan for excessive movement (clutch type) by rocking a fan blade back and forth. There should be little or no play. If your fan is attached to the water pump this is also a good way to check it's operation. Again, there would be little or no play. Despite the popular myth, your car NEEDS a thermostat. Do not remove it from operation. To properly check a thermostat, remove it from the vehicle and place in a pan of hot water of approximately the same temperature (usually stamped on the thermostat - but not always). Generally, if it opens - it's good. Check your manual for the proper temperature range and replace if found defective. You can also check water pump flow by leaving the thermostat out of the housing. Re-fill the car with water and run the engine with the radiator cap off. Once filled, you should easily be able to see a 'current' flow by. If your view is blocked, rev the car once or twice. Water should come flowing out of the radiator at a pretty good rate. Don't forget to replace the thermostat.
If your car stays cool at lower speeds and all of the above checks out ok but your car still overheats at high speeds, it's probably the radiator. With the engine warmed up sitting at an idle, measure the temperature of the upper and lower tanks. There should be (ideally) a 40+ degree differential with the upper tank being hotter (obviously). Even if the overall temperature is not exceeding the maximum heat range, a lower differential indicates a problem and the radiator needs serviced or replaced. Example: If the top tank is at 195o (normal operating temperature) but the bottom tank is a 185o then the radiator needs serviced. Water is not cooling before it heads back into the engine and at high speeds, this will just get worse.
Also , re-check the lower hose. On some cars I have seen this hose collapse after the engine warms up, restricting flow. Even though the hose appeared to be in good shape. There are also some engine conditions relating to fuel mixture (a 'lean' condition) that can cause overheating, but this is rare on any newer cars.