John Williamson wrote

Thames water are losing 693 million litres of water *per day* through leaks in their pipes. (2011 - 2012 estimated
figures), out of a total of 2,600 million litres supplied. That's about a quarter of the supply just leaking away into
the ground.

Other water boards here lose about the same percentage.

And because they do, it just isnt practical to 'fix the leaks'

Leaks are a part of any piped water system.