Well, first off, the ‘why’ almost literally doesn’t matter. The guns are out there, and there is no functional way to get them back. That’s not an issue of rights or of opinions, it’s almost narrowly an issue of math.
And getting them back, as many as we could get back if we wanted to get them back, wouldn’t matter anyway. Analysis of that here:
The Magic Gun Evaporation Fairy
Three Reasons Why Gun Ownership Rates From Other Countries Don’t Matter
As to my personal justification for why, it’s also very mathematically driven, and covered here: