It seems it’s not that easy to ‘simply replace a battery’.
Safety
First and far most: human safety, We’re talking about soft-cell battery replacements. These things are potentially dangerous when handling improperly. Seeing as Samsung was in a WORLD OF HURT after phones starting to explode there is something that could be said about safety regulation and testing. And perhaps more importantly: the perception of proper human safety.
Time Constraints & economics
Let’s say you’re a repair shop and you have someone paying you 100 bucks for a battery replacement. That technician employee costs money to hire and cannot spend an infinite amount of time on 1 single battery replacement. At a certain point it’s more cost efficient to have an employee spend x-amount of time on something and just refund/replace the repairs that take too much time and/or fail for some reason instead of having a continuous economical human repair clock. Especially when it's a small percentage of ALL total repairs.
Customer Service
When is a customer happier? When they heard a technician has spent 2 additional hours straining on a repair and getting their original product back overdue OR when they’re come back at the allotted time with a working device? Though the choice seems quite objective…what if the original has just a single additional scratch?…or an additional faulty free component that got replaced due to complications? It suddenly shifts heavily to the direction of a replacement as this is a HUGE part of Apple’s service strategy and customer satisfaction ratings.
Variance & State of the device
It’s a B*TCH to repair any piece of used electronics but it’s even more of a B*TCH when it’s dirty, used, perhaps mishandled. With any repair there comes a risk of breaking any other component and that risk obviously increases when you’re getting a dusty ass piece of tech. Now couple that with the fact that smartphones are delicate, some of them are waterproofed and need to be resealed, have different tech, have been repaired before, etc. etc. HOLY **** does that create a lot of variance.