Because the Do Not Call list is Government Feel-Good Crap for several reasons:
First I will use a quote by someone else that has summed up the Federal/Government DNC List nicely: "The truth is that the DNC List is rarely enforced. Not because people don't report, but because a report just results in an entry in a huge database of violators. It does not result in an enforcement action per se. The database is available as a resource for law enforcement, but something has to trigger a law enforcement agency to use the resource. In general, this means that only massive violators that will generate some publicity will get any attention." All that said - the DNC List is also available to private citizens and groups for law suits against organizations.
Further, VOIP services has made it where the offending companies can be anywhere in the world with a US phone number, or can be here in the US but can be untraceable or can change their number at will.
Also, there is technology that will allow companies to "mask" or insert a fake caller ID - illegal but since there is no enforcement...
There are also other tricks these people use like going through multiple switches, "breaking into" a telephone companies system and using existing numbers on switches to make the calls out, etc.