A given power figure on a shaft rotating at given speed will give the same torque. This is how it works. You can't really argue with this. So I say again, gear both drivetrains to the same output speed, and the one with more power has more torque

The only thing is, I'm fully aware of this. With his answer Fried validated the initial question, and I still maintain that for normal people using cars normally, they don't require much power at all. They may use it in short bursts because it's there, but I've had rentals with only double digit horsepower figures, and I was never a hindrance to normal flow of traffic.

A tractor probably comes close to using what their engines can output frequently, and if they were to do ploughing a lot faster, power requirements would go up. It's almost all about speed.

But on the flip side, gear down a normal car engine to output the same rpm, and the torque would be higher (with the premise that a car engine has more power).

Cars don't require much power, only the drivers do. Power is king, torque is just a matter of gearing

I don't really need to. Horsepower is the measure of work that can be done. An engine with more horsepower will be able to do more work, hence most car engines could easily do the work a tractor does. And with regards to torque, that's just a question of gearing.

Speed up stuff = do more work.

I don't quite agree, and I would say the premise for the question is false. Cars don't require much power, unless you're going very fast or hauling something heavy up a hill (fast). Farming equipment don't need the same kind of speed. A tractor probably actually needs more power than a car to do what it needs to do