Because traditionally you can't milk pigs.
因为从习惯上说，你没有办法给猪挤奶。
They are one of the animals where the sucking motion and the vacuuming of the mouth needs to happen at once. So 100 years ago you would only get milk out of a sow if you actually sucked her teats.
猪是通过吮吸和吞咽（获得食物）和其他（哺乳）动物一样。数百年前，只要你吮吸母猪的奶头，你就能喝到奶。
And this is only if you could get that close to the sow. Pigs are not nice and docile. They have a mean streak, know what they want and when having just produce piglets and in mom-mode they can be very aggresive.
前提是你要能靠近母猪。猪并不是友好温顺的动物，它们本性吝啬，知道自己要什么，当它们生了小猪，变成猪妈妈以后，就会变得很有攻击性。
Nowadays we could probably build machines that could milk pigs… But we have no tradition of using pig milk and cows produce more milk and much easier. So why start doing something that would be bothersome and of little use?
现在我们可以搞个机器来吸猪奶，但是之前我们没有喝猪奶的传统，而且相比较而言，牛可以产跟多的奶，而且更加容易，所以何必大费周章，自讨苦吃呢？

This paragraph from a BBC blog cannot be beaten as a clear and concise explanation..in short, they are VERY hard to milk…!
以下这段摘自BBC的博客，我要说的更加直截了当，简而言之就是，要挤猪奶是难上加难！
Although pigs’ milk is high in fat (around 8.5% compared to cows milk at 3.9%) and is an excellent source of nutrients, sows are very difficult to milk.
尽管猪奶的脂肪含量很高（差不多在8.5%高于牛奶3.9%脂含量），而且猪奶的营养丰富，但是猪奶很难挤。
They have around 14 teats compared to a cow’s four, and they don’t take very kindly to having them touched by humans. They also get very agitated if you try to restrain them.
比起牛的四个奶头，猪有14个奶头，但你要去伸手触碰，他们会极其不配合。
Pigs also have a limited milk ejection time of around 15 seconds, whereas a cow’s can be up to 10 minutes.
如果你想要控制它们，会招致它们的愤怒。
All in all, it’s a pig-shed load of trouble to milk a porker.
此外，与牛出奶10分钟相比较，猪只有短短15秒。总之，想要挤猪奶，等着你的就是麻烦一箩筐。
Personally.. I would imagine that no one would really want to try that hard..pigs are low to the ground, and in non-wild conditions are notably smelly and muddy!
就我个人而言，我想应该没有人真的要去尝试挤猪奶。。况且猪就贴着地面生活，圈养的环境让它们总是臭气熏天、满身泥污！