Because hi-fi stereo equipment is specialised, quite different from appliances, and requires a specialist approach to fault-finding and repair.
Hi-fi equipment contains unique components and mechanical designs. A unique set of knowledge and skills accompanies working on this type of gear. General electronics repairers, musical instrument techs and whitegoods repair guys do not have the requisite knowledge, skills and tools to expediently and expertly repair this gear.
Many pieces of equipment I’ve worked on had visited general electronics repairers beforehand and usually, it’s just a waste of time and money. Occasionally it’s much worse. One really important aspect of this work is the parts used in hi-fi gear and the tools used to repair it.