right, but the stolen phones get sold in other countries where the carriers don't care if the phone was stolen but care that someone is spending money on their service.
Visit eBay and search for "blocked IMEI" or variants. There are plenty of used phones which are IMEI locked due to either: reported lost, reported stolen, failed to make payments, etc.
I the lines between IMEI banning or blacklisting and the modern unlocking techniques they use have been blurred a little bit and so some carriers and some manufacturers don't really want to do or spend time doing the IMEI stuff and would prefer to just handle it all via their own unlocking and locking mechanisms.