Because, the RAW file (from modern cameras) describes a color-space much wider than AdobeRGB.
The setting that we might assign in-camera applies only to the JPG created by the camera - it has no impact on the “raw” data that’s transferred from the camera’s sensor into the RAW file.
@KeithRJ has posted a link to an excellent article on all this (here) … At first glance, it seems quite dense - but it’s actually not too long and is a relatively easy read.
John M