Because many folks have no record of their iphone serial number.
What watt charger do you use?
I wonder if folks with rapid battery capacity loss is due to a high wattage charger (i.e 20 watt), and charge daily to 100% generating a lot of repeated charging cycles.
For LI battery longevity it is best to charge only to ~80% with a lower wattage charger (e.g. 12 watt). My iphone battery is 2.5 years old and still at 90% reported capacity.
p.s. Battery replacement is relatively cheap, no need to use a new battery as a reason for getting a new iPhone