Because there are so many people who don't understand the basic physics to motion. They think that slamming on the brakes means that any vehicle of any weight going any speed can stop on a dime. They have no idea of how precipitation and ice can change the stopping distances and they think that no one in front of them would ever have a reason to stop.
If you don't believe it, look at this recent thread. The OP was completely clueless about stopping distances and responsibility. The sad thing is that there are many more of these types of drivers out on the roads now than the ones that actually understand the physics of stopping and driver responsibility.
https://www.dcurbanmom.com/jforum/posts/list/1162787.page