Earthmoving trucks have had loadweigh systems for quite a while.
Pressure transducers in the struts and a control (summing) module - not such a great expense.
True. But what does this system do? It prevents overloading the truck and damaging it from overload. Not a great deal of precision or accuracy is required. That's not what the aircraft system must do. It must provide both a fairly precise and accurate total weight that directly affects take off performance. If the truck is too heavy and climbs up a grade or accelerates more slowly than expected no one dies. If the airplane is too heavy and climbs or accelerates too slowly, LOTS of people can die.
And more than just weight is involved. For the airplane, CG is involved. Many of the same weight numbers drive the CG calculation. And unlike the truck, improper CG can kill people on an airplane. (Witness NA Flt 102)