Im actually not super familiar with DAWS but perhaps some actually dont ship these sounds with and rely on plugins? In which case the user can just fetch free/licensed ones to make their music and the DAW doesnt have to ship it.

Maybe I'm undercutting my own point - I thought DAWs typically had these sorts of sounds but now that I think about it I recall hearing that sometimes people use plugins for this sort of thing.

Nah you have the right of it.