OP has some particular type of project in mind, where what they say probably makes sense. Not all large codebases are like that.
For example, it could be a lot of individual small projects all sitting on some common framework. Just as an example: I've seen a catering business that had an associated Web site service which worked as follows. There was a small framework that dealt with billing and navigation etc. issues, and a Web site that was developed per customer (couple hundreds shops). These individual sites constituted the bulk of the project, but outside of the calls to the framework shared nothing between them, were developed by different teams, added and removed based on customer wishes etc. So, consistency wasn't a requirement in this scheme.
Similar things happen with gaming portals, where the division is between some underlying (and relatively small) framework and a bunch of games that are provided through it, which are often developed by teams that don't have to talk to each other. But, to the user, it's still a single product.
For example, it could be a lot of individual small projects all sitting on some common framework. Just as an example: I've seen a catering business that had an associated Web site service which worked as follows. There was a small framework that dealt with billing and navigation etc. issues, and a Web site that was developed per customer (couple hundreds shops). These individual sites constituted the bulk of the project, but outside of the calls to the framework shared nothing between them, were developed by different teams, added and removed based on customer wishes etc. So, consistency wasn't a requirement in this scheme.
Similar things happen with gaming portals, where the division is between some underlying (and relatively small) framework and a bunch of games that are provided through it, which are often developed by teams that don't have to talk to each other. But, to the user, it's still a single product.