Because buying the models even at GW prices is likely cheaper than the time and effort necessary to do a good job of printing them. Model piracy does occur but it takes the from of Chinese recasters making molds directly from the model sprues. That all being said there is a market for this and my guess is the BOM cost of the hardware necessary to make a 3D scanner capable of resolving the detail of a tabletop mini is in the $10-$30 range right now. No way there's enough market to foot the bill of SW dev tho.