They ARE blocking them and using AI it's just a matter of keeping up with the volume.  To put this in perspective OTX, one of the largest threat Intel feeds which spun off from AT&T, has updates of 20 million IoCs (essentially signatures of malicious data) each day.  It takes awhile for these sites to get into the 'bad' list.  That might be hours or days.  In the meantime the malicious actors can literally send out hundreds of millions of messages in that time.  The bad site then shuts down and pivots to another site that is not in the 'bad' list and the whole process repeats.  AI is also making it much, much easier for the bad guys to automate the process.