Geotech is a bitch. Soils aren't homogenous and vary a lot in both composition and characteristics. There are multiple types of TBMs that are designed to operate under very specific conditions, for example a soft rock/soil TBM excavates through different mechanisms than a hard rock TBM. One of the big problems in any excavation is you have no idea what's actually down there until you start working in it. The underlying material can differ a lot over very short distances. You can very easily be minding your own business driving the TBM and run into a transition that will straight up mess you up. Combine that with all the previous mentioned stuff and you get a lot of cost overruns and time overruns. But yea, subsurface is a bitch.
Even if you had ideal conditions or an amazing new TBM that goes through any material, you're still dealing with a machine that scrapes at rocks for miles on end. The parts that do the digging need constant replacement, underground, on the inconvenient side of the boring machine, and even if you did the actual digging faster that'd mean repairing it even more often.
I’m not disagreeing with you (I know nothing about geotechnical), but I was thinking the bottleneck is all the bureaucracy and infrastructure factors, which don’t have mechanically scalable solutions. I’m assuming tunneling through Las Vegas is much different than tunneling in NYC or SF or even LA.
