Because the overwhelming majority of them operate from OUTSIDE our borders and as the laws for "telemarketing" are written today, the Telco's can't shut them down.
Congress needs to act to fix that ... (wait, I need to stop laughing after typing that ..............................................................................................)
Or better yet you should familiarize yourself with the technologies that are out there that can help you protect yourself and your landline. There are options available, some of them FREE.
For example, porting your phone# over to Google or one of the other VOIP services comes with free Telemarketer blocking service.
A friend of mine uses ViaTalk which is less than $8/mo for his home phone service and swears by it. I checked into it but opted to just cancel my home phone/landline and return the number back to AT&T.
“For example, porting your phone# over to Google”
My landline is ported over to Google. They are blocking a huge number of incoming calls, but I still get more than enough, mostly with spoofed local numbers.