im sure if cogman was ever asked why not appear on APOIAF, he would reply with "I've never been asked". the thing is people like cogman usually arent gonna go out of their way to ask a podcast for an appearance, it usually takes persistent efforts on the part of the podcast to get guests to appear. or they might turn down a request because of bad timing/unavailability and just never get asked again. im sure cogman has appeared on other podcasts simply because they happen to ask at a convenient time, and he said yes.

understandably our hosts are often busy with their professional or personal lives to be able to reach out to 'celebrities', but i'm sure it would happen more often if more direct public relations was initiated from apoiaf itself.