It’s an artifact of SmartThings’ cloud architecture (and considered a security feature by Samsung).
Part of setting up the hub is registering it with an account in the smartthings cloud, which is associated with the original email used to set up that account.
Once you do that, a reset of the hub may roll the physical device back to its original shipping condition, but its serial number is still associated in the cloud with that original email.
You’ll see lots of threads in the forum from people asking how to setup a hub that they bought used, and the answer is always contact support.
So it’s just how this platform is designed. 
(As far as the “welcome code“ that was a card in the box for the V1 and V2 hubs, and you used it during the setup process. The OP didn’t say which model hub it is, which is why I mentioned it. And, yes, it was a pain if you lost it. 
Here’s an old thread as an example.