It does. But the spammers still do it. Because eventually they hit one person who gives them a thousand dollars or whatever and it pays off.
Preventing spoofing doesn't have to make spam cost-prohibitive for every spammer to greatly reduce the volume, and it does not interfere with ordinary people obtaining phone service anonymously.
Preventing spoofing doesn't have to make spam cost-prohibitive for every spammer to greatly reduce the volume, and it does not interfere with ordinary people obtaining phone service anonymously.