There are several issues in play here. First is the danger one. Getting electricity wrong can kill and if someone giving instructions leave out a tiny part they can be liable. Sorry, but this is todays society in the USA.
2. One doesn't know what is really on a machine 50 or 70 or 100 years after it was made. If someone posts stuff on fixing it to factory specs the don't know what is in your hand.
3. Problems are often hidden I replaced external wires and a control on a machine and it still wouldn't operate. I knew the wire were bad. I cold see the brass wire. I then opened up the motor and saw bigger difficulties. Replaced it and all was well, but if had put out a bid for the machine instead of working on my own, would have lost money.
4. Some areas get pretty crowded in sewing machine electrical housings. One strand of wire in the wrong place can cause serious trouble.
5. Soldering is a learned skill and many people don't do it well.
6.Some cabinets force you do disconnect wiring to take a machine head out of them Stupid stupid stupid.