Why is there no sweet (as in 'with sugar') butter? I understand salted butter can be useful is some situations, but why there's no such thing as "sugar butter" that can be sold in stores? I mean, there is sweet milk (concentrated, for instance) and sweet yoghurt, but no sweet butter nor sweet cheese. Is it just cultural or is there some chemical explanation behind it?
Butter traditionally has salt in it as a preservative, mainly to stop bacterial growth on the residual whey, and to slow fat rancidity

Modern butter has a much lower amount of salt than is required for shelf storage in the pre-electric era. Now we have refrigerators, salt is not required at such a high level

Butter with no salt has nearly half of the shelf life of modern salted butter

Processed sugar is a modern ingredient, and therefore there is no culture in having it in butter. 

Sweet yoghurt and milk are all "new" inventions

Using salt as a preservative is as old as recorded history. Many cultures used salt to preserve fruits, as well as meats and vegetables