The sell because some want to be able to say they did it. It's not because it's easy, they have great machining skills, or that it's cheaper.
I wouldn't pay more for a rifle with an 80%, I would just about discount the gun the entire amount of a new lower due to concerns with it not being anodized, the quality of the machining, etc. No warranty or potential return for not working right, either. A mag well a tad to0 big or small, issues with parts fitting etc make them less than desireable.
So, if you can buy a finished lower for less, even including the FFL transfer fee, and get one done to milspec, why buy an 80% and do all the work? Notoriety - when you show it off, it doesn't have any markings and you can claim the BATF doesn't know about it.
In that regard, the BATF can't tell you where the straw purchased guns that went to Mexico in Fast and Furious are, either. Moot point whether it has a number or not. If someone is really worried they will come to take your guns, it's more important how you store them.
Oh, btw, someone now sells a 0% lower. Yep, just a big square chunk of alloy. You get to do it all.