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There are very few late night departures presently from SFO, so there is very little demand for restaurants in the evening presently at SFO. Additionally , the COVID restaurant restrictions at SFO have only been recently relaxed. Perhaps when the volume of evening departures increase, the restaurants will resume later hours.

And as answered Polaris sit down dining ended over an hour prior to closing time and the buffet started winding down soon afterwards.

SFO has been one of the most COVID impacted UA hubs.