High school robotics competitions are popular and have been around for decades. Even a hobbyist can build a robotic arm with a 3D printer. However, why is it so difficult to build a useful robotic arm?

I watched the following video three months ago and was shocked:

It just looks amazing, which made me want to build one myself to understand how they work. People say building a robotic arm is difficult, but I didn't know. It's always the best way to learn something is to build it right?