Because they are the rules we've adopted to make society, the markets, contested access to scarce resources, drug safety, or what- have- you work. Sometimes there's more than one set of workable rules, but we have to pick one.

Ideally, because contextually it makes sense. It might not be inherently evil to drive 180 miles per hour, but we might guess it to be statistically unsafe in residential neighborhoods, so we prohibit it.