Because it's stupidly expensive boring a Tunnel when you can just do it on the surface.
Also Railway lines take up considerably less space than every Street. Weather also rarely impacts rail
With tunnels you can also go 3D, you could go 100 tunnels deep - stacked - increasing capacity on the same line as needed; something Elon points out as part of the traffic congestion problem is that cities went 3D (skyscrapers) but traffic did not.
It's such a complex issue that a fully "complete" overview is necessary, that can be then dove into over weeks or months of time to first study its thoroughness to then be able to decide how accurate or credible it is, and add to or correct it if an incongruence or error or incomplete aspect found.