There are many aspects of clothing we tend to take for granted and buttonholes ae one of them.
Horizontal buttonholes are placed on garments where there is likely to be strain. The sideways movement prevents pulling and gaping.
Vertical buttonholes are easier and faster to add to a garment making them cheaper and therefore more common.
If you were having a blouse custom made for you and you were large busted the buttonholes would be strategically placed to ensure no gapping. Not only would they be cut horizontally over areas of strain, the first button would be located midpoint between your nipples and then spaced evenly top to bottom.

If the clothes in your wardrobe are not serving you as well as they should a personal style consultation will help you know which garments and styles will expand and invigorate your wardrobe and your style.