A guide can only watch so many people. Identification has to be confirmed individually, so a walk of twenty is not twice a walk of ten, it is a walk where somebody picks something unchecked.
The calendar is brutally short. Most of the year's revenue arrives across a few autumn weekends, so every cancelled place matters far more than the same gap would in a business trading all year.
Storms happen mid-season. A policy on cancellation, rescheduling and refunds agreed before the first booking prevents an argument during the only weeks that pay for the year.
The group's details matter on the day. Who cannot walk far, who is coming with a child and who has an allergy affects the route and the tasting, and collecting that at booking rather than in the car park is elementary.