Many drivers rely only on mileage when thinking about service, but that does not always tell the full story. If a car is driven rarely, it does not mean that oil and other fluids stop ageing. Time, temperature changes and moisture still affect them.
Short trips are actually fairly demanding for the engine. A cold start means the oil has not yet reached proper operating temperature and lubrication is not ideal. If the car keeps doing short trips, the engine often shuts down before it has warmed up properly.
When a car sits for long periods, condensation and moisture can build up inside the engine. This affects the oil and can reduce its protective qualities over time even if very few kilometres are added.
A good rule of thumb is to follow both mileage and time. If the manufacturer says for example 10,000 km or 12 months, both limits matter. If the mileage does not fill up, it is still wise to change the oil at least once a year.