Seems you don't quite understand how many users register every day. Well over 200.
1 in every 100 ever makes a post on the forums.
1 in every 1000 stays for over 5 posts.
1 in every 1500 stays for over 30 posts.
1 in every 6500 stays for over 50 posts.
The ones above 30 posts typically become an active part of the community for a while, and those who go beyond 50 are guaranteed to stay for much, much longer than that.
It's simply not humanly possible to make a welcoming post for each and every one who registers. If they want to be a part of the community, they'll introduce themselves of their own accord.