I believe the issues lies much more with copyrights than anything. As a developer, you can purchase 3D models to use in your commercial game. However, only to be used within that software. The dev has most likely bought some or many of the assets being used in Flowscape, which each comes with different copyrights.
People often sell a small set of game engine ready for about 20-50$ or more. You can't honestly expect to have all those assets to use within you own unity/unreal projects for 15$.
And why does 3D models, rigged and ready to be used in game projects costs as much? Because it takes hours to make.
I suggest looking into tutorials for open-source softwares for 3D-modeling like Blender. There's also many free textures to use online.