If you're expecting scammers to abide by the Do Not Call registry, you'll be waiting quite awhile. The scammers are quite often located well out of the reach of the US government. The phone numbers are nearly always spoofed (These scammers figure what numbers within an area code are not currently in use and spoof them or their use a prefix that is invalid.), so it's very difficult to report them properly. The various means that are at the disposal of the average person to address such calls are not 100% effective and if these scams didn't work, they wouldn't be running them. The more people that thwart them, the better, but not everyone will use the tools at their disposal.
The government does enforce the law in regards to the Do Not Call registry, as there have been companies that have been fined for violations of the law. The problem is that the bulk of the ones running these illegal scams make use of various technology to make it difficult to track down. It's illegal under Federal law to spoof the Caller ID info in order to defraud someone (This primarily came about because of some unscrupulous debt collectors posing as local law enforcement and even making the Caller ID info look that way.), and the ones that seem to do it these days are the scammers. VOIP services are part of the problem as well, because some of these scammers are located in another part of the world yet with the use of VOIP, they can make it look like they are located where they are targeting people. It's tougher to track these scammers down because it is a very tangled web to unravel. Criminals don't care about the laws they are breaking.