Game developer here. Typically in 2D games, there's two kinds of polygons: sprite polys and collision polys. In Unity (and probably many other engines), a sprite is an object with a square (4 vertices, making a square polygon) and usually another square poly for the collision of that sprite. Often, a 2D game will have objects that use a long series of connected vertices to make its own sprite, which creates a complex polygon- and the same goes for that object's collision poly as well. Point being, there's a lot of flat polygons in 2D games.