Why do my ICNS files converted from PNG have a flat white background instead of transparent?

In Preview, you can set a default image background color in its Preferences > General panel. When you have an image loaded with a transparent background, you will either see a transparent checkboard if the View menu > √ Show Image Background is selected or the default opaque background color of that menu item when it is not selected.

Hi race bannon, 

Thank you for using the Apple Support Communities. We understand that when you convert an image file in Preview the file has a white background instead of a transparent background and we would like to help! To try to resolve this problem you will need to try to remove the white background, follow the steps found in the Apple Support article Extract an image or remove a background in Preview on Mac. 

We hope this helps.

Take care.