The short answer to why we cannot build a central hub is:Time.
The time to do that would have been 30+ years ago. But if anyone had guessed that a massive hub was needed - I don't think it reached above back room talk. The only focus was on the big airports and when Heathrow was not expanded (ask the politicians etc.) the focus went to: LGW (additional terminal and squeeze capacity), STN, LTN, LCY, eventually SEN.
Simultaneously, the regions found their options and so we have had 25 years of expansion in (variously): BRS, SOU, BHX, MAN, LPL, LMA, NCL, GLA, EDI and probably more.
So, to change that? You would not only have to locate the new hub but persuade all folks who use the other airports (pax, carriers and staff) to change to your idea. Not to mention that all the plans for road and rail connection to the new super-hub would have to be put in place. Lastly, all the folks who use the regional airports (pax, carriers and staff) would be mighty upset that they would see a reduction in services. That is because KLM/AF, Lufthansa, Ezy, RyanAir etc, would have to review all their operations and consider what to move and what to cut.
You can see how that will not happen.