Wow, the attitude...Do you even know what Avisynth does?
All encoders are just a wrapper for x264. Handbrake does the least, and it's why people who are new to encoding use it. There's nothing wrong with that, but it doesn't have all the features power users need.
I would say people use Handbrake because it's free, well supported, works great, strikes a good balance between highly customizable and ease of use for a typical enthusiast who just wants to rip movies without achieving a graduate degree in video encoding sciences.