Because it makes the QR code simpler (less "pixels") and therefore more robust. Given the same physical size, scratches affect it less, and a phone can read it from further away. You want to avoid long links in QR codes whenever possible.

The other thing is analytics, of course. You can generate 10 different small QR codes that lead to the same long URL, to find out where they're getting scanned, so you know which locations are popular and worthwhile and which ones are not.

And maybe don't criticize restaurant owners for being technically incompetent? I'm sure they would find you just as incompetent at cooking at scale.