You've missed the legal / regulatory side of things.
Apparently this has just recently changed, but telcos are highly regulated in the US and they are legally required to execute calls placed by their customers. This means that they have to be very very sure the call should be blocked before doing so, otherwise they face legal liability. This regulatory structure means that call spamming in the US is all but legally protected.
Compare this to less-regulated email, where Gmail and other providers are free to block spam based on any reason: source ip, domain, content of the email, etc.
Apparently this has just recently changed, but telcos are highly regulated in the US and they are legally required to execute calls placed by their customers. This means that they have to be very very sure the call should be blocked before doing so, otherwise they face legal liability. This regulatory structure means that call spamming in the US is all but legally protected.
Compare this to less-regulated email, where Gmail and other providers are free to block spam based on any reason: source ip, domain, content of the email, etc.