It's one thing to hammer one house together. Spending a career doing it is another matter.
I spent the first few years of my carpentry career framing, siding and trimming without the advantage of nail guns.
Time is one aspect of the gun advantage, but there are more benefits.
Using a gun tends to give a much cleaner, more professional end product with less splitting and zero hammer tracks.
With proper training, guns are much safer and result in fewer injuries. I've never shot myself with a gun in over 20 years of using them.
In the few years I spent hand nailing, I lost my thumbnail so many times it doesn't even hurt anymore.
I also had chronic joint pain in my fingers, wrist, elbow and shoulder when nailing by hand, not to mention lower back pain from being folded in half for much longer to complete the same task.