Perhaps someone who understands the ad industry can help me understand it a bit better. Does Google ban forums because there's simply too much inventory around

Perhaps someone who understands the ad industry can help me understand it a bit better. Does Google ban forums because there's simply too much inventory around otherwise? I realise the stated reason will be that it can be unpredictable what people might say on forums and such, but that doesn't feel like the "real" motivation. Any user generated content (eg. comment sections on articles) could result in ads being placed alongside unsavoury content.