Eating by the seasons is also pretty interesting, I think. It forces you to expand your gastronomic horizons, explore the cuisine of different regional cultures. Some cultures don't use milk (and thus cheese or butter), some don't use much oil, some are vegetarian while some are nearly all meat. There's preservation by fermentation, by drying, by salting, by burying, by sealing in hardened butter. Some just eat a lot of soup. There's really an infinite number of dishes that express flavor, aroma and texture. If you ever get bored of your food, you can fix that.
I suspect this has to do with space and weight constraints, and probably a touch of old-school procurement practices.
In the not-too-distant past, basically everything was flown to south pole station, so weight was at a premium. Powdered milk weights a lot less than UHT milk. Now they do a traverse to the pole with sleds and tractors, so weight is less of an issue, but volume might still be.
On top of that, procurement may be slow to change. If, in fact, weight is no longer a constraint, it might take years for procurement to change to include buying UHT milk.
But more practically, there are different types of containers for UHT milk hermetically sealed to different extents. The more expensive kinds can last years. The US Military says 10 months for "normal" UHT milk stored under "normal" conditions [0]. The more expensive kinds can go much longer.
[0]: https://www.dla.mil/Troop-Support/Subsistence/Operational-ra...
Best by dates for shelf stable/frozen food are often not safety related, so the antarctic program just charges forward with whatever they have.