How is phone call spam a revenue stream for carriers? Most plans these days are unlimited calling. Under that assumption, spam calls are cost and bad customer experience.
On the other end, if they were to try to stop spam calls, it would cost money. You'd have to have a way to find out if a caller has legitimate business or not: A collection agency might be marked as spam quite a bit, yet shutting them down would likely mean a lawsuit. Sales calls seem to fall into a weird grey area. There would need to be a system for consumers to report spam calls. These all cost money to build and maintain.
Not to mention enforcement, which would be especially tricky given international calls and the bit about legitimate businesses being marked as spam.
They probably also don't have your usual private phone plan with unlimited calling but some sort of high volume call centre plan.