(This is a US-based response response about the US phone system.)
Because it's a political problem, not an engineering or business problem.
In order to prevent monopolies, we have very strict requirements about carrying all calls that enter the network. Otherwise, phone companies would randomly block each other as part of anti-competitive tactics to push each other out of business.
The telephone spammers take advantage of these laws, because it is very difficult to legally block a spam call. Your phone provider must carry all phone calls without any prejudice!
(So I'm about to get very close to the line about hacker news's "no politics" guideline. Please consider that spammers "hack the law" and "hack politics" before you flag me. Instead, I would appreciate it if you take a minute or two to fact check my opinion disguised as fact.)
The primary way out of this is by a legal change, not by an engineering change or culture change: We must update our laws so that it is very difficult to make and carry spam calls on the telephone network.
I personally believe that one way to do this is to require that all phone calls provide rich metadata to the phone that is ringing. I really should know who is calling me, and the phone company that originates that call should be responsible for verifying it. I would even consider a requirement that makes it trivial, or automatic, to "lift the corporate veil" on who owns the company that is calling me. Otherwise there should be strict penalties for a phone network that allows originating calls with phony or spoofed metadata.
(I also don't think anyone has a right to make a 100% anonymous phone call. I think we can figure out how to allow people who need to make anonymous phone calls to make them, without exposing the entire phone network to SPAM.)
Ultimately, every phone call that enters the phone network needs to have a strict and auditable chain of liability.
In reality, if our laws included a requirement for strict metadata, and then a requirement to make an "abusive phone call" button really obvious, we could figure this out pretty quickly: at this point it becomes a simple reputation system.
Finally, we need laws that prohibit co-opting the phone network for marketing. We may want to start with prohibiting unsolicited commercial and political calls. We need to make sure that there are no loopholes, where the company you bought something from 2 years ago is still free to call you as if you have an ongoing relationship. We need to recognize that pulling out one's phone and looking at who is calling is really a significant burden to us, and that our government should protect us from companies and politicians who think that they have the right to interrupt our day at any moment to listen to a sales pitch.
If you really care and want to put time into this, advocate politically. (And remember that hacker news is not supposed to be political.) Don't waste your time reading instructions about how to report spam.