Maybe on the surface (pun intended) it appears stupidly expensive in comparison, but I bet if you actually put a value to all of the positives then the cost would at minimum be at par.

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.