If you think it's simple you don't really know it.
It's not that the supposed concepts that are hard, it's that it has one of the messiest interfaces and set of documentation ever devised for a software.
And there are hardly any settled concepts, the terms it used came to be as it was developed, and can have different definitions in different parts of the documentation.
If you consider it simple I'm very seriously concerned about your software engineering output
If git was aimed at regular peope I'd agree it's not well designed. It's not. It's a tool for software engineers to do a job. It is not flawless. But it was massive improvement over the code management tools that were in use before it and I haven't encountered a replacement I prefer.
A lot of the problems people have with git are actually problems with bad workflows. You can argue that git should support arbitrary workflows but that's like arguing a garbage truck should support offroading.
It's not that the supposed concepts that are hard, it's that it has one of the messiest interfaces and set of documentation ever devised for a software.
And there are hardly any settled concepts, the terms it used came to be as it was developed, and can have different definitions in different parts of the documentation.
If you consider it simple I'm very seriously concerned about your software engineering output