40%rifle,40% reloading skills, 20% shooter...imo
I'm going to agree this is close to the right answer....
With your indulgence, I think the outcome is based on 1/3 ammo, 1/3 rifle, and 1/3 shooter.
Every fall, there are small towns in The Rockies that have sight-ins for hunters who show up from far and near.
This means the sample size of folks is pretty big and there are a variety of guns, ammo, and shooters. Probably about a quarter or third of them show up with hand loads, the majority are shooting factory ammo.
Some small percentage show up dialed in and are proficient, the vast majority is split between mediocre and a disaster.
I can probably count these proficient folks on one hand for a given day. But that means there are dozens who suck bad, and the majority that are clearly not practicing or loading enough. Half those proficient shooters are shooting factory ammo, so handloading isn't the only factor.
I give folks credit for their enthusiasm and for showing up, but probably less than one in five are up to the task of hitting a kill zone within the range of western elk/deer hunting. It explains the dead/wounded animals found in the landscape and the poorly shot ones you see at the processing plants.
On several occasions, we work on the rifles and hand them back and they do just okay in the hands of their owners with their ammo. Could be the travel shifted their boresight or loosened their rigs. Most will say they left home on boresight and are surprised they are now off.
Once in a while, I will take their gun and my ammo and I shoot and their guns shoot beautifully, then hand them their corrected rifle with my ammo.... A few will suddenly drill the target, but most will still not shoot a group.
Even if we only consider the folks who show up with hand loads, the above comments still hold. If we assume they were loading good bullets with proper workmanship, the majority still can't shoot well, so it isn't just the ones shooting factory ammo.
The point of the story is easy. It takes a good rifle, good ammo, and good shooter, to get a good outcome.