More specifically, triangles cannot be curved as a plane, they can only lie flat; any 3 points are always co-planar where as 4+ can be messy… As a result, wrapping textures across an always-flat surface is faily easy and not prone to warping/errors. This is why everything is devolved into triangles when making a mesh. A sprite, at the minimum being a square, needs two tris to be complete. One might ask why not just the 1 tri, I don’t know that…