If I made a ghost gun and I transferred it to a friend or relative, why whould I need it to have a serial number or undergo a background check ?
There is no such FEDERAL requirement. Your state may have different laws.
HOWEVER, and this is a big 'however' if you made that gun specifically to be transferred or sold, then you are breaking federal law. You may only make guns for your own use. If you LATER decide to sell one or give it away, then that's not a problem, but if you are making guns for other people you need to be federally licensed and follow the applicable laws if you don't want to run afoul of the Federal Government.
Just in case it's not obvious, my above comment is about how things are, not about how I think they should be.
How do I think they should be? Well, it's complicated because to make the situation ideal would take a lot more than just changing the gun laws.
In very short form, I think that for the most part, things/objects/substances should not be regulated but actions should be. There are some exceptions to that, but this was supposed to be "very short", right?
For that to work, people have to be held responsible for their actions and penalties must be severe enough to have significant deterrent value for non-offenders and to prevent recidivism in offenders. I don't believe in rehabilitation for intentional violent crimes. If someone intentionally and criminally attacks other members of society, they need to be heavily penalized, and repeat offenders need to be dealt with in a way that completely prevents re-offending whether that means termination or permanent incarceration.
Right now we find ourselves in a situation where a very small percentage of the population is terrorizing the rest of society and a lot of that has to do with the two facts:
1. Offenders are allowed to re-offend.
2. Penalties do not provide a sufficient deterrent.
Here's an interesting study.
https://www.sciencedaily.com/releases/2013/12/131206111644.htm
"One percent of population responsible for 63% of violent crime, Swedish study reveals"
The study found that 4% of the population committed all of the violent crime, and that a quarter of those offenders, only 1% of the population (made up of reoffenders) was responsible for 63% of all violent crime.
By the way, I do not believe for a minute that our society is willing to take the steps necessary to establish a level of personal responsibility and punishment sufficient for my idea to work properly.