It's not really. It seems like it is because it has a baroque CLI with a ton of commands and those commands all have a ton of switches and the same commands can do lots of different things.
But under that atrocious CLI git is very elegant and simple conceptually. Once you understand the underlying concepts (blobs, trees, commits, refs, index/stage) the rest flows naturally.
Now I've been working with it since its earliest days including contributing to it here and there, but I never really found it very hard to grok.
"Git from the bottom up" is dated but git hasn't changed conceptually since then. "Git for computer scientists" is another good one. But any of the tutorials that start with git's low-level concepts and build from there are where I'd start.