Whilst i understand your sentiment vis a vis certain engineers skill sets etc the supply chain is place to supply trade, not joe public.DIY does not make non professionals idiots. In fact some diyers may actually do a better job because they are not being paid by the hour. I don't disagree with your comments about more technical systems, however, surely the manufacturer has an obligation to ensure that the fitter is advised how to correctly install the panel. what if the fitter wires it up wrong (which pro's no doubt can and do do) and buggers up the panel, the manufacturer cannot rely on the installer not having followed the instructions should the panel be returned as faulty.
Even gas cookers come with an installation guide, despite the fact they have to be (or should be ) installed by a Gasafe registered tradesman!
Margins, stock, WARRANTIES, rebates, customer loyalty, tech support, pricing etc etc.
And yes, i do know of instances where the moron behind the counter sold kit to the public who just wanted it cheaper than the trade installer was offering fully fitted. - Trade guy was one of our accounts and we took the initial tech support call. Only the one mind, and a very short one at that.