When you offer a contract on open tender you force consultants to compete to get the contract. They are willing to undercut each other on price and offer to do the job as quickly as possible. If the project was performed in house, there would be no competitive bargaining process, the employee is the monopoly provider. In this case the employee extracts some surplus by taking longer than is efficient to do the project.

In a sense we have an agency problem. The government employees know that their effort is not observable, so take longer to finish a project. By hiring consultants you get past this agency problem, as they are forced to join a competitive market to get the project in the first place.