I know you do not want to hear it, but SMS is a cellular service. Apple can only control messaging on their own software which is what they do with the Messages app. The same is true for encryption. The SMS standard that the cellular provider adopts does not support end to end encryption and just because it is sent to an Apple device does not mean they Apple can encrypt the message. That is something else that Apple can only do with the Messages app. In a perfect world, there would be no SMS messaging and it would not be a problem.
Who would they report the SMS message to? The phone number is easily spoofed, so it would do no good for Apple to keep a database of junk numbers as some of them may be legitimate numbers that would be blocked. Apple has no way to validate a senders phone number as they can only receive the messages that the cellular provider sends to them.
Using Messages, Apple is able to validate the sender. If a sender has been confirmed to be sending Junk emails that do not follow Apple TOS, they can remove the Apple ID and the sender will no longer be able to send messages. It is much easer to spoof a telephone number than it is to create several fake Apple ID addresses, since the email address must be validated before you get an Apple ID.