How comes you repair guitar amplifiers etc but not hi fi units, surely an amplifier is an amplifier?

Unfortunately there are several factors restricting repairs, mainly cost to replace with new as opposed to repair costs, we’ve also found many HiFi brands use hybrid components built specifically for the unit and once they’ve built a batch production is stopped and so getting spare parts is very hard. We also struggle with schematics and drawings for the repairs again boosting the cost trying to diagnose blind.