Why do trucks have so many gears?

Diesel engines have high torque but low rpm operating ranges, and carry heavy loads.

The range of gears allows the truck to get moving with a load and bring it up to speed utilizing the best operating conditions of the engine.

That way a reasonable engine size can be used, to operate like a passenger car the engine would have to be the size of the entire cab of the truck, and would use an outrageous amount of fuel.