If you're above ground you'd just pop an emergency hatch in the tube.
You can't do that underground.
I'd also point out that a hyperloop tunnel is a much worse place to be stuck in than a normal tunnel.
In a normal tunnel there is room to walk alongside the tracks.
In a hyperloop tunnel there is virtually no room between the car and tunnel - maybe an inch or two, tops.
This also applies to any sort of failure that compromises the vacuum. A dipshit with a rifle could kill anybody in the system in a fraction of a second. Burying the tubes is probably the only way to avoid this, but thankfully the whole thing is vaporware anyway.
Humans can survive (uncomfortably) at .5 atm and you eliminate the vacuum cannon problem simply by putting pressurization valves periodically throughout the tube.
I am not sure what do you mean by 'ubes are not air tight and actually breathe', but they definitely are not air permeable.
I'm just extrapolating from trends and some cherrypicking. The ongoing elevated rail project in Hawai'i is now running at $500M per mile. Boring Company projects they can get cost of tunneling down to $10M per mile.
What I forecast (without much data, so please correct me if I'm wrong) is that with more money going to tunnels, we will see the costs continue to drop. You don't have the quasi-extortion that goes on now over land rights above ground.
Tunnels also scale better long term—you can just keep adding tunnels.
But beyond cost and scale tunnels are just so much nicer. You can replace ugly, polluting above ground infra with parks and greenery. Even though the Big Dig in Boston took forever and went way over costs, the city is so much nicer now without the Central Artery.
Note that you're (or, rather, Elon Musk is) comparing apples and oranges. Tunnels are actually cheap, about $80m / mile. The expensive parts of metro systems is not the tunnels, but the things that go inside them and stations. This is doubly so for the inexcusably high costs of US transport construction. When people talk about the cost of metro systems, the divide the total cost of the expansion (or entire system if it's a new one) by the length of the system, so they include the amortized costs of new vehicles, infrastructure inside the tunnels, and stations (which often cost more than the tunnel to get to them! Station excavation isn't cheap).
Also, all the materials used on the Hawaii rail line have to be shipped in - literally, adding expense.
China builds a mile of track at half the price because they literally pay their people half the price (or less). It doesn't matter that their local Purchase Power might be greater.
Taking it a step further, the Parent was comparing a mile of track in China to a mile of track in Hawaii, which literally has to ship all supplies, and likely a lot of labor, across an ocean. So it's not a great comparison even if we remove labor costs.
In practice, laws are not always followed to the letter, but the same can be said in the US. You have the right to appeal to higher court, which is effective as unless it is a hugely major project, expropriations are generally made by the municipal or regional government.
As for environmental studies, they are directly paid for by the government. Environmental standards obviously vary, but have been strongly strengthened in the recent push against pollution.
It matters because it means that the economies are not directly comparable.
You said they can only do that "without paying US prevailing market wages to the contractors." as if that is somehow an unfair advantage. But if their workers don't need US salaries to have a good living standard, because purchasing power is higher, and are not being exploited, where does the problem actually lie?