Some anecdata. I get a few dozen spam SMSs in any given month, as well as some spam phone calls (varies a lot by month - sometimes only 1-2, sometimes 20+). I have received a total of 2 spam iMessages (I remember them because they’re notable to exist), and 0 spam FaceTime audio calls, and I’ve had this phone number associated with iMessage since it was first released.

Regardless of the reason, there is substantially (many orders of magnitude) less spam on Apple’s networks, at least for me, when compared to SMS/RCS/telephony.