Is this all BS or does it make sense to you?
To me, it's psychological; triangular sandwiches always look more appetizing than rectangular ones.

Why do triangular sandwiches taste better than rectangular ones?

Several studies have confirmed this phenomenon. It seems that triangles give you more sandwich filling per bite. A triangular sandwich has two 45° corners that allow you to bite much further towards the middle on your first two bites, where there is more filling. This is followed up by a perfect, crustless third bite as you take the space left in the middle. Bite number four is, of course, mostly crust, but you can’t have everything!

[sciencefocus.com]