Remember, all of those "Lifetime Warranty" parts are "Lifetime Warranty to the original purchaser."
That's all well and good for a bunch of old kooks like us, who will keep a car for 20 years, but, statistically, most people who are buying parts at the local parts store and doing the work themselves have older cars, and most of them won't keep those cars more than another year or two before they sell them or junk them. You can bet that A-Z has those statistics honed down very finely, and their "Lifetime Warranty" probably only has to make it past 12-24 months in 95% of cases.
– Eric