Because 3D libraries like OpenGL and DirectX draw everything using triangles, including sprites.

You put a texture (image) on triangles, put two triangles side by side to make a square and that's how you draw a "sprite" with OpenGL or Direct3D.

Because 3D libraries like OpenGL and DirectX draw everything using triangles, including sprites.

You put a texture (image) on triangles, put two triangles side by side to make a square and that's how you draw a "sprite" with OpenGL or Direct3D.

The sprites are drawn using triangles, its not for collisions, its simply for drawing them.