Because it’s an absurd idea. He’a trying to solve an already solved problem with an inferior consumer solution. When you look at the commuter trains of the major metro areas you’ll notice some very well off people that could afford to drive, but choose to ride in a comfortable train and focus on reading or catching up on email. Driving is an inferior mode of transportation. And before you yell, “but driverless cars!”

Consider that many folks don’t want to buy a car in some of these bigger metro areas or to pay for the parking to maintain one. For so many reasons, beyond technology, traditional mass transit is the superior option.

I couldn’t care less whether Musk achieves his technological goals with the Boring Company. It’s hawking an inferior product even if he does.

Tunnels are dangerous. Elon is proposing to build really narrow tunnels that are hard to evacuate and then allow every random vehicle inside it. Yes it's on a sled but who is certifying that my fellow tunnelgoers' vehicles won't leak fuel, spontaneously catch fire, or find some other special way to kill me?

Elon has some good ideas and some stupid ideas, this is of the latter category.

Switzerland has a couple of tunnels in operation that have short haul trains running through them that you drive your car onto. They don't check what kind of cars are on the train, as long as they fit you're good.

Yes, but those tunnels are not as narrow as Musk suggests and have good ventilation and an emergency exit every 100m or so. Source: Swiss that used the said tunnel multiple times.

You also need hardcore vent systems.

The tunnel could allow just Teslas - with their ultra fancy air filters you could set off a chemical weapon inside the tunnel and everyone in the cars would still be fine :D

By the time these tunnels are ready (if ever), urban private transportation will probably be primarily electric anyways.