This noise adds considerable to the background noise people in towns and cities have to endure. Like car alarms people tune it out reducing its effectiveness. However the noise spreads over a wide area, e.g and on building sites this beeping carries on all day. This type of noise carries over a wide distance.
Warning people that a vehicle is reversing doesn't absolve the driver for their responsibility and people with hearing difficulties would not be aware of it or young children know what it means.