Just to share one datapoint - I have not looked at the internals myself, but having talked to a couple of devs who has spent a great deal of time working with Blender at a code level, one takeaway seemed to be that there was a lot of code duplication caused by each dev doing their own thing and not having an overall architect to rationalize some parts of the code.

A particular example that was mentioned was the importers that were each done standalone rather than built on a common framework. This was a couple of years ago and of course things may have changed since then.

Just to note that the developers were still extremely positive about working with and in the code, but it does seem plausible that there would be issues like this in a codebase where many devs focus on a small piece of the overall puzzle.