The primary reason is not the pinblock. In fact, many harp makers use piano
pinblock stock for harp arms. The main reason pianos stay in tune so well is
that they are built around a cast iron frame. This has about the same thermal
expansion as the steel in the strings, and is rigid enough to keep everything
in place. The weight, on the other hand..... When was the last time you saw a
concert pianist schlepping their own instrument to a gig?
I once built a hammered dulcimer with a steel frame. It was heavy because all
I could get for the frame was mild steel tubing, but it sure holds it's
tuning. Now, a proper truss of 4130...