Except that contractors don't, for a variety of reasons. Contractor A may not be successful at a second project where the requirements are compatible. Extracting and storing a TBM against future needs is a crapshoot. Transporting the TBM away may have its own technical challenges - depending where the extraction shaft is located, roads or waterways may not be helpful. As noted the technology may be changing, so a new and faster TBM may be a better economy for later projects than a slower but used and paid for TBM.
I do know of one project in Ontario where the original plan was to simply drive the TBM forward and bury it. It didn't happen, but that would havve been the cheapest and simplest solution.
If you want a lot of detail on TBM's, try
here.
PS - the case for reuse is also being made, see
here - but the authors acknowledge this is against conventional thinking.
- Paul