2) Weather. Inclement weather limits when you can drive on roads and how fast as well as reducing safety dramatically and reduces capacity.
3) Speeds. Can't safely go 125mph on a road unless you have a LOT of space to accelerate. Even on the autobahn, this is pretty dangerous.
Tunnels are 3D and do not take up space from other things, deconflicting the tension between single-occupancy vehicles and buses (and other modes of public transport). And small tunnels (gas, water main, electrical and telecomm conduit, storm sewer, and sanitary sewer) are already ubiquitous, going under basically everyone's property and in multiple levels. This is especially so in large cities, when the size of those existing tunnels can approach what Musk envisions.
If you could somehow dramatically lower the cost of tunneling (and tunneling in particular, not trench-and-covering, which disturbs the surface), then you could do transport much more efficiently.
New York City shows what a great subway system is like and how tunneling enables efficient transport in spite of huge densities. But it's 100 years old, and for whatever reason (labor costs, safety, etc) we simply could not afford to build another system like it nowadays.
Today, when buses get their own lane, service reliability increases for the buses. What you could say is that the current system of roads benefits single-occupancy vehicles and that this, as a private solution, would force people to pay the true costs. But quite frankly there will likely be subsidies for this, especially because Elon Musk seems to view this as a way to reduce car travel times for himself, which leaves us with the same problem as we had before.