This implies that the issue is due to being open source. The majority of closed source software, especially in house software, has minimal or nonexistent documentation. If there is any, it was likely done once at the start of the project and never updated since.

I've been a consultant for over 10 years. I always make sure to ask for access to any documentation for systems I'll be working on. I think I've gotten significant (out of date) documentation maybe once. This isn't an issue stemming from being open source.