Because your cars run on gasoline and would have filled the tunnels with choking fumes, without either (1) a big expensive ventilation system, or (2) expensive electrified rails that would...impose friction costs?
For 2) you can use magnetic levitation: Inductrack which now manifesting out as Skytran. (Costs are about 1/50th your transportation mass in magnets.)
For 2) you can use magnetic levitation: Inductrack which now manifesting out as Skytran. (Costs are about 1/50th your transportation mass in magnets.)