Because they have failed to use their noggin to think such scenarios thru ahead of time. They have become lazy drivers that think it is just as natural and comfortable as walking. They have forgotten the inherent risks associated and the amount of responsibility they carry when driving. Our laws and society has allowed it to happen.
Many accidents are caused by negligence. There are millions of accident scenarios and it is impossible to think of all of them.
No law or society can prevent accidents. To think so is negligence in itself.