The U.S. constitution has been written in an age before phones, automatic and semi-automatic rifles (at least in common use), nuclear weapons, high-bandwidth communications networks that operate at lightning speed, mass media, unbreakable encryption and CCTV cameras.
But since having 300 million people have a detailed, nuanced discussion about anything is impossible, everyone works at the edges.
Specific examples for the UK: inducting politicians into the Privy Council in order to qualify them for security briefings, Henry VII powers, and ministers' authority deriving from the seal they're given by the sovereign. Which would almost make as much sense if it were a marine mammal as it does being a stamp.
The thing being, they work well enough. And if you want to replace them, you need to work out what to replace them with and how.
Hey, I really like this framing. This is a topic that I've thought about from a different perspective.
We have all kinds of 18th and 19th century legal precedents about search, subpoenas, plain sight, surveillance in public spaces, etc... that really took for granted that police effort was limited and that enforcement would be imperfect.
But they break down when you read all the license plates, or you can subpoena anyone's email, or... whatever.
Making the laws rigid and having perfect enforcement has a cost-- but just the baseline cost to privacy and the squashing of innocent transgression is a cost.
(A counterpoint: a lot of selective law enforcement came down to whether you were unpopular or unprivileged in some way... cheaper and automated enforcement may take some of these effects away and make things more fair. Discretion in enforcement can lead to both more and less just outcomes).