> I don't see why software projects can't be the same.
Contractors can't automatically give subcontractors access to the plans, the construction site, and so on. The contracting agreement will have provisions about who can access the site, how they get approved, on what terms, and so on. Becoming a construction contractor doesn't give someone carte blanche to invite other people to a site without regard to the terms of the contract.
Likewise, a software contractor can't just decide to share your codebase and design documents with a subcontractor because it almost certainly violates many of the contractual clauses they agreed to.