There is only one legitimate reason to have an SN# on a firearm and that is so you can report a stolen gun in hopes it will be recovered. I can’t think of another good reason.
I’m not familiar with building your own AR, but your comment leads me to ask this...
The stripped lower receiver has, I think, a number on it... correct?
This seems to make it not a ghost gun. Are their lower receivers that do not have numbers?