For two reasons.
1. Most manufacturers recommend a time interval as well as mileage, mostly due to sludge, acid and condensation
2. Lightly used cars - and 1000 miles per year is VERY light use - have in fact some of the toughest conditions for motor oil. Typically these are not driven hard enough, long enough to get the motor out of enrichment (dumping gas into the motor); or to get hot enough to boil off acids and water that are natural by products of combustion. Consequently the oil will tend to have very hgih dilution and acidity, and generate more sludge and deterioration of surfaces.
I do UOA on my street and track cars. One would assume the race car would have more oil deterioration, dilution etc. But not so - its perfect with nearly undetectable dilution and low acidity. The street car, on the other hand, is not so lucky.
I might even want to change the oil MORE often on a car used that little. I would also make a real effort to, when it is driven, drive it long enough to have it at full warm up (above 200 deg F) for at least ten minutes.
Is it good oil capable of lasting longer? Absolutely. If that the limiting factor? Not in my opinion.
Just Me