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

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.

You build that route through nothing. There is no need for tunnels, and I don't see any positives, that you couldn't remediate much cheaper. For animal movement you can just build huge animal bridges every few hundred meters. Weather is rarely a problem for train operations, tunnels create also a whole lot of new "Negatives". You need evacuation systems, ventilation systems, maintenance etc...

I really would love to see an effort, perhaps best spearheaded by Boring Company themselves, to do a very thorough "feature matrix" of pros and cons and real-life current costs of things depending on the area, e.g. snow removal costs, how much building "huge animal bridges" that allows ecosystem to not be too disrupted, etc.

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.