Touchscreens can be made at massive scale and then repurposed in batches for everywhere. They’re always the same (roughly speaking). Buttons are individual components, you have to lay the whole thing out custom how you want it to be, you have to put all these fiddly little components together… just having a robot make a big square object along with 199,999 other ones is cheaper, even if technically the big square object is orders of magnitude more complex than the chunks of plastic and springs and buttons etc.