Why don't people do this then? In the U.S. in most states, leaving children unattended even close to home for any amount of time is considered a form of neglect or abuse, though the precise wording varies by state and the age of the child.
The risk is that if the police pick up your kid or if the state department of child and family services catches wind of this, they can take your children from you and you have to go through a very unpleasant process to get them back, after which there are mandatory home visits by the state.
Sadly, this happens more than it should since the laws in most states are very vaguely defined and the police and state employees at the department of child and family services are given extremely wide latitude in these situations. In some states the precise age at which you can leave children unattended isn't even explicitly defined; the interpretation "too young to be left alone" is left to the police or the state agency that handles these types of things.
The most common victims are the working poor who sometimes will leave children unattended because they have to work and either can't afford or can't find childcare.
It's a really bad situation on many levels, and for some reason there hasn't been a serious effort to reform a lot of these issues. I know the whole thing must sound absolutely crazy to anyone outside of the U.S. but it really is how things work in most places here.