The problem was that they didn’t want the extra hassle of verifying that senders owned the numbers they were announcing. In the earlier SS7 era that was manageable because all of the parties were major phone companies but VoIP opened up a wave of small fly-by-night players. Porting the system forward without recognizing that change in the security assumptions was recognized as a mistake in the early 2000s but the telephone companies saw preventing it as a cost which would also reduce their revenue from delivering all of that spam.