Since the 80 lower is an un-serialized receiver blank, it can not accept a trigger, hammer, or safety, therefore it is unfinished firearm that's not operable and can not be made to operate without additional fabrication and because of this ATF does not recognize this as a firearm.
Here is the actual purpose of 80 lower receivers.
There are few good advantages that come with building your rifle with an 80% lower instead of just buying a stripped lower for a custom build.
It doesn't require to be serialized. In-fact federal law clearly states that a firearm made for personal use doesn't require any serialization. Although the state laws might vary and some states might do require serialization for blank receivers before fabrication.
The FFL isn't required to purchase one unlike stripped receivers or firearms. You eliminate going through a gun DEALER or FFL. Since 80 lowers are not FIREARMS, you can actually ship them at your door steps.
There is no BACKGROUND CHECK needed to buy a 80% lower.
And they are more affordable in-terms of pricing and privacy of what you have ownership of which is your business only. So that's pretty much it.