Now I think promo threads on the forums are kinda bogus anyways since most of us are also creators, but anyways-
I think the reason people keep making them is that promo threads have a 'expiration length.' People only dive so far in promotion threads to look so really only the top posts get clicks.
Threads like the official milestone mega thread are link graveyards at this point because nobody checks out the later ones, and I think people realise this hence why they constantly make new ones.
However, I think milestone threads are really pointless. What would I care if a comic or novel has 30 or 300 subs? And why people have to do hyper specific ones like 40 subs or so? I get 100 subs but still, check existing threads please.
I think if you are going to make a promo thread, then atleast make it an interesting topic like genre, or something other specific people would actually look for. Like if people are looking for action comics for example, that's where a promo thread could be used for.