Because it's actually simple to understand. There are different forms of complexity and one of the worst is when someone else has built something and you are trying to figure out how it works. Like Fedora, Ubuntu or whatever.
With arch you install the system yourself so you know what you have on it. Therefore problem solving is a lot easier. You put data into the config files so you know what you did.
People run their arch systems for years and never reinstalling. Sure, you will have some bugs but the arch wiki is amazing to help you fix them.