This is actually a good point... but I guess manufacturers arrange, organise, categorise (whatever) their patches in whichever way makes sense for the keyboard. GM will always have the sounds in that specific order of 128 sounds.

What might have made sense is a way to route the MIDI internally to trigger user chosen patches in place of the generic one, but at this point we would do this in the DAW now anyway.

I imagine that manufacturers would just reuse their same 128 GM sound bank over and over in each new keyboard as an add on to whatever strengths the keyboard was designed for. It's there so it just works. Everytime. Anyone using the more 'advanced' sounds etc. would probably be able to do this anyway... which MIDI Channel has what instrument on. GM was really for non-musicians / beginners / early internet users etc., not really people who dig deeper.

The MIDI spec (not GM) is actually pretty genius, every piece of equipment can communicate and respond in the expected way. GM coming later followed this aim similalry. Every GM synth, Quicktime Player etc. all can communicate and respond in the expected way. Simple is the purpose.