Earlier quoted context omitted.
(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 advanta…
Of course, that would require the politicians to first opt their own political calls in to the existing (if largely useless) measures such as the Do Not Call registry.
Notably they can't use autodialers without having prior consent from you.