Disclaimer: I have not really looked into simulation yet. I plan to do that around Christmas.
Another big reason are concerns regarding licenses and patents. There is a reason why there is a huge cost difference between simulation tools with and without model libraries. (There is also a reason why a lot of the manufacturer models are encrypted.)
Especially models that model more than the simple stuff will be quite hard to come by under any reasonable license.
My assumption is that ngspice already comes with such simple models. (If this is true than we might want to point probable contributors to their contribution site. Adding models there will be even more useful than adding them to kicad alone as this will profit everyone using ngspice with whatever frontend used.)