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.