I'm not a figure modeller but I heavily use the Genesis 8 digital human from Daz 3D [0]
My guess is that teeth haven't been that interesting to date and that the modellers don't have access to hundreds of reference pictures / geometries of teeth for their modelling. As opposed to reference images of skin textures, etc. Tongues are probably the same. In fact I don't think I have ever created a render of a figure with their tongue out.
Once they take the time, the teeth should be easy to improve.