When every bid comes in far above the estimate, the problem is usually the estimate — and the consequence is a re-tender that costs more time than the original estimate saved.
A reliable engineer's estimate is built from measured quantities, current market rates, and an explicit contingency, each traceable to a source.
The common shortcuts are using last year's rates without escalation, omitting site-specific access or logistics costs, and applying a nominal contingency without assessing the actual risk profile. Each of these compresses the estimate below what a contractor can price.
Where the estimate and the bids diverge sharply, document the reconciliation before deciding to re-tender. Sometimes the market is telling you the specification is unbuildable at the quantity stated.