Hmmm, good point and also the semi truck diesel engines only go up to 2,100 rpm redline and have lot of low gear ratios. There are even some 60 feet articulated city buses with a 4 cylinder diesel engine, too. Not all of them, some have 6 some have 4 depending on what option they wanted.
Also, diesel engines are more energy efficent than gasoline engines, they burn more of the fuel, thus more efficent power to move a vehicle or whatever it's cranking.
