To make all of the code available to everyone is not a good practice, even worse if access is granted r/w.

There are security and IP aspects that the company must consider and granting access only to what is needed by each person is a standard good practice in that regard.

On a more technical note it also forces people to maintain well-defined APIs because these are the only thing customers (e.g. other teams) can see.

Keeping the source code for different products on different repos enables to achieve this encapsulation and also decouples all maintenance aspects.

In practice there is a sweet spot to find. In a microservice architecture people can get trigger-happy and create many, many services to build the product. Having a repo per microservice can become cumbersome very quickly.