Tunneling underground is extremely expensive. Individual self driving pods aka cars are very expensive. But most importantly cars do not get more efficient in capacity just because you put them underground, plus now you have to build entrances and exits to the underground system everywhere. Traveling also becomes a lot less appealing when you're in a black tube all the time rather than seeing the city around you. It's also not possible because the underground is not just unoccupied but there is already other existing infrastructure down there.
What you are describing is just a much less efficient, worse version of a subway.
> cars do not get more efficient in capacity just because you put them underground
I don't think the entire "car tunnels" thing is reasonable either, but this one is wrong. The bottleneck for cars is the inherent interference in the 2-dimensional streets. If you shove them all in a few tunnels right until they get into a low-transit region close to the destination, it would increase their capacity by a lot.
It's also a huge amount of money that is better used some other way. But it has an effect.
Surface streets aren't inherently 2D, though - you can build overpasses and underpasses and above-grade highways. We just don't, because it's hideously expensive.