Because they fit in the gap between the proper brake lever and it's body. To make that gap, you have to slacken the brakes off otherwise they'd be stuck on all the time. In other words the only way of fitting them is to make the brakes less powerful.
And then there's the other problem, that they're really long and bendy. So the brakes are already not as good as they should be, and you can't even pull harder on them to make up for it because they just start bending. Using them to stop in a hurry is suicide.
They were only invented because numpty americans would hold the wrong part of the handlebars and then ride straight into the nearest wall. Apparently making a new law was easier than learning how to ride a bike properly. :facepalm: