Comment Triangles are Easy (Score 5, Informative) 230
Graphics cards use triangles (not general polygons, but usually triangles) because they are easy to render quickly. All the GPU needs to do is an easy 4x4 matrix multiplication per vertex to get the screen space coordinates, and rasterize the points that fall inside. (And in most cases, the vertices are shared and indexed between multiple triangles, thus less than vertex transformations are required per triangle.) Moreover, the transformations are highly parallelizable, as the GPU is doing the same matrix multiplication to a large number of vertices, simultaneously and independently.
A triangle mesh is a linear approximation of an arbitrary surface. You can get arbitrarily closer to that surface by subdividing the triangles. And if you make that high enough, you are well below the pixel size. (FYI, this idea of "micropolygons" was the basis for Pixar's original Renderman software.)
It's important to realize that most games and other 3D applications are not geometry-bound, they are fill-bound. A significantly larger amount of time is spent on shading. For example, assume that you have a scene with 10 shadow-mapped lights. This means that for every pixel that is ultimately rendered to the screen, the GPU performed *at least* 10 lookups into a shadow map to compute lighting visibility, and then evaluated and summed the reflectance function for each of those (each of which may itself involve multiple texture lookups). And depending on the draw ordering, it may be the case that those evaluations were wasted, because the fragment ended up being occluded by a later-drawn fragment that was in front of it. Multiply that by the number of pixels. And if you're using supersampled antialiasing, then multiply that by the sampling rate.
There has been a lot of work on non-polygonal representations of geometry. In the early 2000's, point-based rendering was a very active area of interest (search for "QSplat" for a prominent example). NURBS (or parametric surfaces generally) have long been used in modeling applications. There are also plenty of examples of implicit surfaces or voxel-based rendering. But given the rate at which GPU speed has increased, it is often faster and easier to just use more triangles.
Of course, there are things that are not well-represented by triangles. As noted, a triangle mesh approximates a surface. If you have something that doesn't have a surface (fog or smoke, for example), a non-triangle representation could well be preferred. But for the most part, the objects that we want to render can be sufficiently approximated with triangles.