A traveller arriving at a French restaurant in mid-afternoon usually finds the kitchen closed. The gap between services is structural rather than a matter of local habit.
A restaurant runs two services, not one long one
A kitchen serving both lunch and dinner performs two distinct pushes, each requiring preparation beforehand and cleaning afterwards.
The break between them is when the second preparation happens: stock is checked, deliveries are dealt with, and the evening's components are made.
Without that gap the evening service would either begin unprepared or require an entirely separate team, which most restaurants cannot support.
The working day would otherwise be impossible
Kitchen staff who begin preparation in the morning and finish cleaning after the last dinner covers are already working a long and split day.
The afternoon break makes that shape legal and survivable, and working-time rules in many countries require defined rest within a shift.
Restaurants that serve continuously usually do so with two teams, which requires the volume of covers to justify the additional payroll.
Lunch itself is a defined institution
The midday meal in France remains a genuine sitting rather than a snack, and workplaces and schools have historically been organised around it.
That produces a sharp, predictable peak, with most tables filled within a short window rather than spread across the afternoon.
A kitchen sized for that peak has no reason to stay open afterwards, because the demand simply is not there once the window closes.
The set lunch exists because of the same rhythm
A fixed-price midday menu with limited choice lets a kitchen prepare in volume and turn tables quickly during the peak.
It is usually the best value a restaurant offers, since the same kitchen and ingredients are being sold at a lower price to fill a predictable rush.
For a traveller this makes lunch the efficient way to eat well, provided the timing is respected rather than approached casually.
What stays open in the gap
Cafés and brasseries generally serve simple food throughout the afternoon, since their kitchens are built for continuous rather than peaked service.
Bakeries, charcuteries and markets cover the same gap, and in many towns assembling a meal from them is the local solution rather than a compromise.
Reserving for lunch and arriving within the first half of the service is the reliable approach, since kitchens stop taking orders well before the doors close.