> documentation tends to be for users.

There is user documentation and there is technical documentation. Both are very different. Unless you have a lib, framework or such, in which case it's more or less the same.

> but if you're going to be tracking changes in git anyway

Yes, it's a technical system for tracking changes, not documenting those changes on the developer-level.

> so people don't have to hop to a different system to get a quick understanding of the commit?

Because for any meaningful complex product you need to do that anyway. commit-messages are just the bare minimum of ways to document. Though, of course, you can make more use of it, including it in some semi-automatic changelog and such. But usually it's not worth the hazzle.