By the time you must have a subway, because you're built up so densely, and you population's so large, the cost of the disruption require to build, puts them out of reach.
But they don't rust and fall down, and get in the way of new development and have to be removed (and replace with something). And do we all love our elevated highway? Elevateds have all those failings. While subways continue to run in tunnels first dug long ago, elevateds with trendy names, get built, become obsolete and get demolished.
So you invest now in the best future you can, or cheap out and stay cheap for a long time to come.