Musical instruments (normally) vibrate in the sfundamental state. To change the frequency ( and therefore the note) from a guitar string, the player will pin the strings down to force nodes in different places, thereby changing the wavelength.

Piano wires are struck with a hammer and produce only one note each. The piano itself has a fixed size and filling it with wires of varying lengths up to several meters long is not feasible. What is different about each piano wire to allow them to have different resonant frequencies?