[jtolmar]

Great answer, but I also want to add:

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.