Maybe because there are relatively fewer options for free modeling software, compared to the tons of free 2D graphics programs one can choose from? Putting money into Blender helps those companies getting rid of buggy, expensive 3D software. For 2D graphics, they already have plenty of options.
I was able to use Paint.Net in place of GIMP. Paint.Net is Windows-only but it handled pretty much all my raster image editing needs and unlike GIMP I can actually remember how to use it despite only using it every couple of months.
I didn't just mean free options. There are still many other acceptably-priced and acceptably-licensed graphics programs besides PhotoShop. These may still be more acceptable than buying into the PhotoShop ecosystem. With 3D software, it's not so simple.
