Because it's anti social and going outvfor dinner is a haptic experience.

The main reason is that using a smartphone isn't available for the elderly and any other person having difficulties using one.

Also - you typically scan a QR code so have no advance knowledge of what the web page will be. It could even be malware if someone placed malicious QR code stickers on the table.

Why do I need a fucking computer to order food when the waitress is gonna write it down on paper anyway and give it to the kitchen. It’s stupid.