when an external / vendor team picks a project...

estimates = contract amount = project budget ...

it is hard for a project team to negotiate later

often the estimates need to be competitive or bottom most for you to win the contract

no one acknowledges the unknown and risks at the beginning of the project

writing down more than 4-5 risks along with the bid amount is taken as a sign of a team that will not be easy to work with and fight over every bullet point whether something is in scope or a change request

and often the one who estimates and wins the project may not be on the actual project team with delivery accountability