Because the Do Not Call Registry does not monitor, screen,  or block anyone's calls for them.
Go to the FTC/DNC site and read.  It explains the DNC and its function.

This number called me five times today, but I did not answer.     CPAP- cell phone from Freemont, Nebraska.  It is harassment.  Why do I still get these calls when I'm on the Do Not Call Registry?

they are calling about a fictitious health condition. Looking for e-mail addresses, date of birth, etc...soungs like a scammer or phishing...

they keep calling. i have asked them in the past to stop. They called twice today despite asking them to stop,

I've gotten calls from this number at all hours of the day, and since they never leave a voice mail, I don't answer them.  Seeing these comments confirmed my original feeling that it's a telemarketer or a scammer.  Thank you!

I was called by this number today. They said it was about a CPAP machine. They wanted my birthdate, email address, and insurance information. When I refused to give the email and insurance info they ended the call.

It's NSA (no, really)

This number called me twice yesterday & twice this morning. Didn't answer since I didn't recognize the number. I recently re-entered my phone number on the "Do-Not-Call list" & obviously that's not helping.....

number keeps calling me, with an ID showing Cpap (googled: says it about sleeping machines, which ive never used or ordered), doesn't leave a voicemail pretty sure is telemarkerer, my suggestion to everyone, block the number.

This number called a few times, I never hear anyone at the other end.

This number keeps calling me from Nebraska but I am not getting a response or voicemail. I'm assuming it is a telemarketer.