In this case, it's not necessarily sunk cost. There are switching costs associated with bringing in a new contractor, so it is likely cheapest to continue with the current one, even if they didn't bid it right initially.
Switching costs are massive in construction. The permits and loans involved are often tied directly to the contractor and sometimes their subs for licensed trades. Many contractors wouldn’t want to come in and take over at a late stage because they can’t really vouch for what’s been done so it will be seen as risk.
In this case, it's not necessarily sunk cost. There are switching costs associated with bringing in a new contractor, so it is likely cheapest to continue with the current one, even if they didn't bid it right initially.