I personally noticed a lot of project just from signature campaigns.
Ok, you personally notice a lot of project. But do you really invest in these projects?
Alternate cryptocurrencies > Announcements (Altcoins) sections is getting too much post and ANN topic at once to followed them. If you check the section, you may notice that ANN topic is moved to second page in less than 30 minutes by the new posts in other topics. But just ~15% of the project have signature campaigns and bigger visibility.
Yes, you are absolutely right about ANN threads moving to 2nd page in 20 minutes. This is also caused by bounty rules, where participants are encouraged to post on daily basis at least 5 posts per day on ANN thread.