There are many words and phrases for which there are no sources describing their origins. When digital television started in the UK, I needed to get a converter. I went to a shop and bought a box. It was called a set-top box. I put it on top of my television, just like the picture in the instruction book. That's why it's called a set-top box.

Nobody is going to write a book or web page that answers the question "why is a set-top box called a set-top box?" because, to an English speaker, the answer is so obvious - it's a box that goes on top of a set. A mountain-top shelter is on top of a mountain, a roof-top garden is on top of a roof, a set-top box is on top of a set.