in jamf the list of available computer models is stored in the computerModels.xml file located in /WEB-INF/xml/ folder in the jamf war file.
This means that if an organisation wishes to use jamf as a central inventory to store other devices types, windows chromebooks etc, in jamf as unmanaged devices then every jss update needs the file modifying every-time on each jss that the api write may occur on.
If the data for custom non-Apple devices, and possibly even Apple ones, were stored in the database they would stay between upgrades. This would also allow the values for all machine types to be read by the api if required.
Example information for some HP machines.