The conversion happens offline and nothing online can see it
Between a website visit and a meal there is a walk, a drive and a decision that leaves no digital trace.
Somebody looks at a menu, decides, and turns up two hours later. Nothing connects those events, which means standard conversion measurement is close to useless in this business.
The consequence is that restaurants either measure nothing or measure the wrong things, optimizing for whatever the site can count rather than for covers.
The measurable proxies are worth using even though they are imperfect. Direction requests, phone calls from the profile, reservation completions, online orders and menu page views all correlate with visits.
The offline half can be asked about. A question at the table or on a receipt about how somebody heard of you is crude and it is the only bridge available.
Accepting the limitation is more useful than pretending it away. A restaurant that knows its measurement is directional will make better decisions than one treating a website metric as a proxy for business.