Why not??
Because the people writing the law won't stick to their own mandate, and that supposed mandate is false, to begin with.
First off, what is the entire stated benefit of the check in the first place???
To identify people who have a legal barrier to possessing a firearm, so they cannot buy one, and thereby, cannot do harm with one.
Do you agree??
So, what's the practical point (other than irritating people) of running a check on a person who already has a gun??? None I can see. If they are intent on causing harm, they already have a gun (or a dozen) so no check can prevent that.
No check can see into the mind of someone who hasn't done anything wrong, yet, so it stops nothing there, either
I'm not saying a background check does no good at all, anywhere, what I'm saying is that the very best it can do is less than we are being told it will do, and currently its clear that the system isn't working even close to the very best it could do.
And, to make matters worse, we don't get just a law that covers buying from a dealer, we get laws that cover all "transfers" and they can be so badly written that it is nearly impossible to get a clear understanding of what does, and does not apply under the law.
Want to loan a friend of 20+ years whom you know has no criminal background a gun? BOTH of you have to go to the FFL and pay for the check done on him. Then when he gives it back, another trip to the FFL, and another $$ so he can return your property to you!!!
Going to work for the day? Guns at home, all locked up like a good boy?? Does your wife have a key to the gun safe? or know the combination???
Did you take her to the FFL and pay to have a check run on her, when you "transferred" all those guns to her possession when you left for work that morning?? If not, you might be a felon!!
Do you do it every morning before you leave for work?? If not, you might be a felon!!!
I'm not kidding, one law I've seen is written so badly that a case could be made for prosecuting what I've just mentioned!
Its not that I object to the base idea that people who shouldn't have guns, shouldn't have guns, its that the background check idea is being sold as a panacea, which it can only fail at being, and its being done so poorly that the many of those it should stop aren't being stopped and those it shouldn't even be applied to are being punished with inconvenience, and costs that they don't deserve.