It is because for some models, the battery is sticked on the aluminum case. To get it off is risky, Apple avoids this, hence replaces the entire case. It does not make sense to me as well, but it is what is. It was 209 euro charge, and was worth it considering a new outer case, a new trackpad, a new keyboard, and a new battery as the bundle.
Ok, €209 isn't too bad for such a job. That is unfortunately the problem with these later models, you can't easily fix just one part. I am lucky in that the 2011 MBP is really easy to open and fix, I also installed a bigger HD and an extra 16 GB RAM. I still have an iPhone SE 1st gen and it's battery now barely lasts half a day. I considered changing it myself, but it is a risky job, much like your laptop battery. Sending an old phone like that in for repair isn't worth it. So that phone is now a permanent 128GB media player/GPS in the car and I have a new 12 mini.
I really hope you get your machine back, these Apple products are collectors items, in my view. My oldest device is an iPod from 2010 with iOS 6, still works, when plugged in. I don't sell or give my Apples away.
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