Because that idea is BS, designed just to block 3rd party repair shops from doing quick repairs leaving you with the same (or worse) level of service that Apple itself offers.
Why on Earth would anyone need to provide SN of the phone to buy consumable such as battery? Makes no sense at all. Even the display is stupid because most of the time people come to the repair shop is because Apple takes 2 weeks & wipes data from device. Which is kinda problem for many people since they often don't have another $$$ for spare phone and also forgot to backup.
Almost any other vendor lets you buy parts such as LCD or battery and just swap it. And for most of the phones you can do that kind of work in under an hour.