Because about 90% of drivers seem either to be unaware of it or just ignore it and, as a pedestrian, it's fucking irritating! I particularly object to being beeped at when I've already started crossing and a car subsequently swings round the corner.
"Rule H2 of the UK Highway Code mandates that drivers, motorcyclists, horse riders, and cyclists must give way to pedestrians crossing or waiting to cross a road into which or from which they are turning. This rule, introduced to enhance pedestrian safety at junctions, applies to both major and minor road turns."
I'd be interested to know who wasn't already aware of this.