It usually contains significant amounts of milk protein, which contribute to flavor but spoil about as quickly as milk does. Washing the butter thoroughly until well after the water is clear will improve storage time, as will salting the butter and thoroughly drying it.

Doesn't that remove the very milk protein that makes it taste better?

Yes, it's a balance between flavor and storage time. If you plan to use it immediately, unwashed butter is best.

But milk does not spoil in 3 days. Why would the butter?

Natural, organic milk does. What you are probably used to is pasteurized, and treated with short bursts of heat. Since at least 20 years, for almost anything milky which you can find in the refrigerated parts of stores. I'm not talking about the stuff which doesn't need to be cooled, until opened, that's heated even longer, and pasteurized harder.

What? Raw milk doesn't expire in 3 days either. More like 2 weeks. And butter unlike cheese has no problem being made from pastueurized milk, so I'm not sure why you'd bring that up.

I've brought that up in response to milk doesn't go bad that fast. Which is against my experience. Maybe I should have that defined more precise?

Under which storage conditions? Refrigerated? Check. Closed container? Check. Climate? Any time of the year, central Europe. Check. Any time of the year somewhere in the Rockies, on the 'Western Slope', at 2600m altitude. Check. After 3 to 4 days it begins to smell and taste different. After which I won't touch/consume it anymore.

I'd be really interested in the stuff lasting 2 weeks, and the conditions under which that's possible?

edit: Again, not that highly pasteurized, homogenized, otherwise treated stuff, but fresh from the cows udder (let's call this really raw milk, which isn't on shelves anywhere, AFAIK), or only the slightest treatmeant, like 'fully organic/bio', which nowadays has a refrigerated shelf life of something like 2 weeks, there aren't any other options anymore. It's all treated. And that stuff still goes bad after opening in a few days.

When proper raw milk starts to go bad, you can keep it at room temperature for a day or so and get something similar to yoghurt. It was done all the time when I was little. I grew up on a farm; the milk came from another farm in the village by the time I had been born.

That may be the case, but isn't what I meant to say, which was just the (refrigerated) shelf-live of the stuff for drinking, and preparation of other stuff, assuming drinkability of it.

Secondary usage of it for other stuff is another matter.

All I know is that I used to buy raw milk from a local farm and it lasted about 2 weeks in the fridge until it tasted bad. Google suggests it lasts 1-2 weeks. I keep my fridge colder than the recommended settings.

It doesn’t. People have been doing this without refrigeration for a long time. The above poster did not wash off the buttermilk at the end which would cause it to spoil.

When unrefrigerated, they used copious amounts of salt to keep butter from spoiling. So much so that you'd have to first wash the butter (wash the salt away) before using it.

I'm not sure it does. It seems to last similarly to me when I make my own as long as I make sure to use sterilized containers to make it and such. It isn't as long as margarine which is maybe the comparison? Not sure.