well the thing is, headsets last the life of a bike normally
bottom brackets on the other hand don't
with threaded bb's you can get all the tools you need for under 50 dollars (wrench, bb tool, etc.)
for these pressfit systems, its well over 100
basically aside from fork/headset stuff, I believe everything should be installable or servicable at home
provided the frame has been appropriately prepreped (i.e. faced+reamed)
i dont think its the standardization thats the issue though. its just that the tolerances are too small and poorly done and the tools are too expensive