1. There's no effective signing on phone calls, so spoofing a phone number is easy. This makes it impossible to prosecute spammers.
2. There's no way to filter a phone call based on the content of the phone call before it arrives. It's possible to spoof senders on email and text messages as well, but it's also possible to filter these based on content before they reach your inbox. This isn't perfect, but it greatly reduces spam in text-based mediums.
You can eliminate ~95%+ of junk calls from the telco side.