Adding on to this since I love everything about it, because other than water, banthas seem to be one of Tatooine's main agricultural products, it stands to reason that people born and bred on Tatooine are very much part of a dairy culture (as in, a culture that produces and consumes dairy products, not something that makes yogurt, though Tatooinians almost certainly also do that LMFAO). An obvious analagous irl example would be camel pastoralists living in MENA, and those people consume A LOT of camel milk, especially when there aren't very many other forms of water/moisture available. This is actually true of most pastoralist cultures living in arid environments (think Central Asia/Mongolia, East Africa as well). Because along with being calorie-dense and nutritious, milk contains a lot of bioavailable water per volume compared to other food, which makes it particularly useful in arid environments.
One of the main reasons humans domesticated ruminant ungulates in the first place is to convert the nutrients found in low-quality forage we aren't able to digest as humans (grass, woody shrubs, etc) into something more readily available for our phisiology. In arid places, this can include converting the moisture found in these plants into something we're able to drink! (Milk is the main product used for that, but also blood in certain cases, particularly in times of deprivation). I imagine the Sand People, since they don't appear to utilize vaporators all that often, subsist almost entirely on bantha milk and black melons in order to meet their daily water intake.
But the Larses are moisture farmers, you might say -- they of all people would have water to drink, they don't need to buy milk to meet their hydration needs (I'm assuming they buy it, since there don't seem to be any banthas on the homestead lol). And, well -- sure, I suppose. But water is also THEE cash crop on Tatooine, and you don't want to be drinking all your money, especially when it can be sold/bartered for a bunch of different, perhaps less important for basic survival but still equally useful stuff. I imagine blue milk is actually probably a little cheaper than water, not quite as hydrating but still nutritious and full of other good stuff like sugar and fats and proteins and micronutrients, all of which are especially important if you're living marginally in one of the harshest environments a human can reasonably survive in. Why only eat your calories when you can also drink them and also recieve a hydration boost in the process? So milk and dairy become not only a survival tool, but also a part of culture and identity. So of course Luke is going to grow up drinking blue milk probably every day, and crave it as an adult out in the big wide galaxy even if he has no intentions of ever stepping on Tatooine again after RotJ.
It's easy as westerners to dunk on Luke's milk drinking habits because, lbr, Luke Skywalker at first glance looks like was plucked right off the Santa Monica boardwalk circa 1977 and most of us who eithet look similar to him or come from western backgrounds probably stopped drinking dairy regularly when we were kids. It's objectively very funny to think of a grown man going into a bar and ordering a tall glass of milk to take the edge off a hard day. And you know what? It probably isn't as deep as I'm making it out to be.
But given what we know of where and how he grew up it makes complete sense for him to be drinking milk way into adulthood. Look at rural Mongolia for example, another extremely arid environment where pastoralism is their primary means of agriculture, 50% of the calories consumed by an average person during the summer come from dairy products (got this figure from the Max Planck Institute), a huge chunk of which is from liquid milk products (albeit probably processed in some way, most likely fermentation). Like, their flagship alcoholic beverage is made by fermenting mare's milk, though it can also be produced at low alcohol contents and drinking this version is a pretty regular occurrence. Like obviously the proportion of milk consumed in the average modern Mongolian diet goes down the more urbanized of a lifestyle they're living (trending toward the globalized mean), but like every Mongolian, along with the occasional Kazakh or Kyrgyz, I've ever seen who's commented on the subject on places like reddit agrees that drinking milk of whatever animal as an adult is super duper normal lol because it's simply part of the culture.
So. All this to say that the SWars writers consistently making Luke order blue milk in bars as a jokey joke is accidentally good and compelling worldbuilding and makes sense given his home planet and upbringing along with just being a cute quirk LOL. Anyway.
