I mean, it's pretty easy to guess:
They want to run the spare parts division at a profit, they know they've got no competition (except used parts off recycled phones) and like some car makers, they know that means they can charge through the nose.
Wikipedia tells me [1] Samsung released 17 distinct model numbers in 2023, and 30 in 2022. Presumably a lot of those have custom, nonstandard screens with rounded corners and curved edges and camera notches and folding and fingerprint readers and whatnot. So presumably the minimum order quantity is huge - they can't just hold a few spare screens and make more on demand, they probably have to buy (and pay for) several years of spares upfront.
And of course every phone will have multiple parts, different subassemblies (do you want to order the screen with or without the fingerprint reader?) and maybe different colour schemes as well. And any spare parts you make and don't sell are wasted.
So it's very easy for a spare parts division to be very inefficient, if it's not well managed. If they're determined to run the division at a profit - that inefficiency gets passed right along to the end user.
[1] https://en.wikipedia.org/wiki/Samsung_Galaxy#Release_history