I don't understand it, why do we have to make our reputation separately on Stack Overflow, Server Fault and Super User even if the accounts are linked?
You're a professional baseball player and are considered the very best in the world, now would that also automatically make you a good chess player, a proper teacher or a physicist?
The same goes for the SE quartet, your expertise on one of the four subjects doesn't necessarily make you an expert on the other three subjects. Therefore, reputation or "trust by the community" on each subject should be earned separately.
Even though it's not always just, deep down inside you will probably trust a 10k rep user over a 1 rep user.
Imagine this (unreal) example: an incredible programmer from Stack Overflow who knows nothing about viruses recommends to never install virus protection because it will slow down his development kit... He has 50k rep on SO and therefore will also have 50k on SU... would you want a 50k rep user to give answers like that on SU?