If everyone trusted everyone else's security you would go through security at the start of the journey and that would be it - just like travelling the US.
Trouble is the UK thinks that it has better security than the rest of Europe so insists on rescreening everyone.
The USA doesn't trust anyone else (unless done at US overseas pre-clearance) hence the shenningans when arriving at ATL or MCO.
If you are flying to Australia or the US from HKG you must undergo secondary screening as they obviously don't trust primary security.
Australian domestic security allows (or allowed when I was there) liquids through security so rescreening was necessary for international.
If everone could agree one standard and international audits ensure that it was met we could have one worldwide sterile area. It won't happen for a while. However there are plans to introduce in Europe (UK I think excluded). Have a look on the Zurich airport website to see what they are doing.