Two fixed commitments a day is the ceiling — one in the morning, one in the late afternoon — with everything between them left open. Three or more booked things per day means the whole schedule collapses the moment one runs late.
Each booked item has a fixed start time, so a delay in the first one cascades through the day. With two commitments there's slack between them. With four, a slow lunch means missing something you paid for, and the group spends the afternoon rushing and irritable.
The moments people actually remember are usually unscheduled: the bar you walked past, the market you weren't looking for. An itinerary with no gaps has no room for those, and by day three the group is tired of being managed.