I've gotten to the point anymore that I just buy the cheapest and view them as a consumable.
I have had a John Deere branded battery (that was almost $50 25 years ago) only last a year, while I have had WM or RK cheapos go 6-7 years.
I agree with the overall average being about 3 years.
I used to pull them every winter and put them on a charger.
I gave up the hassle of doing that after I surmised that made little to no difference.
I have found amazingly that Exide L&G batteries last longer for some reason.
Presently I am running a WM battery (that actually says it was produced by Exide right on it) that is on its 6th year and still going strong. Now that I've said that, It probably will die next week.
I'm sure that heat/cold/vibration, etc. play a factor in longevity. However, I can't help but think that QC plays a factor as well.
Warranties reflect that as well. The longest I have ever observed is 1 year, with 60-90 days being the norm.