Because, if built like subways are in the US today ($900 million per mile), that would only give you a few hundred miles of subway. So a few city centers might have better public transit, but the vast, vast majority of the US would not. (Which isn't to say we can't do both if tunneling can be done much cheaper. It's pretty silly to think we, a mostly suburban nation unfortunately, only need high density subways and nothing else. Particularly during a global pandemic with social distancing.)
Additionally, with highway-like vehicle spacing and high capacity stretched Model Xes (as the Boring Company proposed), the throughput is the same order of magnitude as a typical subway line. With denser spacing, potentially even higher.
Anyway, the question is about what Elon's backup plan was. This is it, regardless of whether it works or not. (And there's plenty of reason to think it won't, to be fair!)
Additionally, with highway-like vehicle spacing and high capacity stretched Model Xes (as the Boring Company proposed), the throughput is the same order of magnitude as a typical subway line. With denser spacing, potentially even higher.
Anyway, the question is about what Elon's backup plan was. This is it, regardless of whether it works or not. (And there's plenty of reason to think it won't, to be fair!)