Because it's very cumbersome to work in 3D, for very little benefit. Projection error can be as little as a couple of meters over hundreds of kilometers if you

Because it's very cumbersome to work in 3D, for very little benefit. Projection error can be as little as a couple of meters over hundreds of kilometers if you choose a proper projection (and creating your own conic projection is easy, too.) In exchange for that error you can calculate distances using Pythagoras' theorem instead of Vincenty's formula or whatever. If you have to calculate whether X is in <geometry> for millions of points, which is typical for a GIS analysis, you can do that in 2D on your own laptop or in 3D on a supercomputer.