Trucks/Tractors dont have high power, why ?

If Power=Torque x RPM, then why do the huge tractors have less power despite having huge torque ?

I managed to see one big tractor working on soil (not the ones that work in farms, ones for land on which something is being constructed) and its engine rating plate showed 120 hp @ 1900 rpm. Why so less if torque is too high ?

Because they don't rev. HP = (TQ x RPM)/5250. Trucks and tractors need massive torque, they don't need the engine to rev and make HP. As you can see by that formula, a lot of torque that happens at low RPM means less HP. If the torque is 600 lb-ft at 1500 RPM, that's equal to about 171 hp. Now, if you alter the flow/cam characteristics of that engine so that the same 600 lb-ft torque peak happens at 4000 RPM, that's equal to about 460 hp.

HP is calculated at each point in the RPM band.