Reply All had an episode that explored it: https://www.gimletmedia.com/reply-all/135-the-robocall-conun... One reason is that a law that had existed was overturned as too broad. Another is the lax laws in the USA regarding what businesses can do with information about customers. In the episode they talk about how an app on your phone that is allowed location access may sell that information to a spam caller so that the spam caller can fake the phone number to make it appear to be coming from near you.