Why is it that an XBox Series S /X SSD CANNOT be replaced?

It's because the Series S/X are new gen. They require the xbox SSD. You are using an HDD. SSD's are way faster than an SSD, the series s is trying to run the 120fps and higher graphics. Thats why they are telling you to move the games. It wants you to play them on the internal SSD storage.

The SSD like all other the parts can only be serviced at an authorized Microsoft service center

We cannot assist in self repair

Are you using a Series S/X?

Series S/X optimized games can only run from the internal SSD or an expansion card made specifically for the Series S/X.

An Xbox One model had a hybrid hard drive/SSD. Now currently the Xbox Series X|S only has a SSD drive in it.

Hi,

All of the games either designed specifically for Series X/S or Xbox One games have been enhanced/optimised for Series X/S must be run either from the internal drive or the Storage Expansion card.

This use of the SSD isn't specifically down to the frame rate as that is handled by the processing and graphics hardware. The reason why they must be installed to the faster SSD is down to the way next gen games are decompressed and loaded into the system.
A specially designed SSD is the way this is achieved, what Microsoft calls 'Xbox
Velocity Architecture'.

All Xbox One games will benefit from Series X/S hardware with more stable frame rates and performance, but Xbox One games that have been specifically re-coded and optimised for Series X/S must be run from the faster SSD drives.