Source code is important intellectual property and trade secret.
In general it is simply good practice to only make this sort of information available to employees who need it to carry out their jobs. It is not a problem because there is usually no reason to study the code of another product/module/team/whatever, and if there is a reason then access can be granted, and is granted.
If your side needs to interact with another team's code and you cannot see their source code it at least forces everyone to have properly documented APIs.