Why is there no sweet (as in 'with sugar') butter?
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