The USA's implementation of caller ID does not require telephone providers to verify that the caller ID provided to them is real (prior to this order).
It also lacks the regulatory structure to "trace" (prosecute/fine) calls through the various interlocking copper, cellular, and internet telephony networks, even when each provider in the chain has data.
It also lacks the legal experience in knowing how (and the political will) to prosecute an individual who dials random phone numbers with an app on a cell phone using a prepaid SIM card and conferences whoever they're calling into a spam line to disguise the spam line's phone number.
Finally, our regulatory system is captive to politicians who depend critically on spam calls for political purposes, and continue to fight to have those calls exempt, making it vastly more difficult to stop all robocalls because some are legal and others aren't.