At my last job, however, everything was locked down and you had to request access to just about every repo. It was a pain in the ass and there was no real valid reason to do so.
I'm struggling to find a reason within my org as well.
They actually don't, sadly. Most companies limit the risk using non-competes.
It's very hard to secure code. If someone wants to steal it, they can usually figure it out.
It's much easier to limit their legal ability to profit from stolen code. Most companies don't have IP that's very valuable on the black market. If you stole Zoom's compression algo, for example, it would be hard to profit without openly starting a new company and violating your non-compete.
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.