Many buttons also means many more pieces to physically install, and many many more wires. And each one (or small cluster) is often accompanied by even more ind

Many buttons also means many more pieces to physically install, and many many more wires. And each one (or small cluster) is often accompanied by even more independently-wired small information displays (small LCDs and LEDs for showing the state / temperature / etc) which are yet more wires. A touchscreen is largely just a single fused physical unit with ~two cables: a data ribbon and power. Utterly trivial to install and wire up in comparison. The total assembly cost adds up very quickly.