> Does a typical enterprise need a distributed source control system?
Yes, unless you are working on a tiny number of source files. The value is that you can work on entire copies of a code base at a time instead of single files. Git does a much better job than making lots of local copies of source code, passing around big diff files and is a lot less hassle than older centralized source control systems.
> Doesn't the uptake in Github which centralizes this distributed system kind of invalidate its main tenant?
No. Not at all. Github is actually a peer with an a few integrated extras for managing tickets and requesting your code be merged into github's repo branches. Most of the centralization is around access control and automation (i.e. continuous delivery, unit tests, etc...).
> Adding complexity without value is a mistake imo.
The main value of GitHub is reducing some operational complexity. For a small 1-4 person team, it may be of little value, but for larger teams, access control, issues, and automation can have a lot of value. For really small teams, fossil is actually quite nice. That said, there are a lot of great alternatives to GitHub that give you similar features.
> Maybe it's my own fault I haven't seen the value...
Probably not. I did the first 20 years of my career without source control and was able to build some pretty big applications. I do think dvcs was a big improvement, and I'm glad it exists now.