Because Apple computers don't use nVidia cards, and nVidia no longer makes their own Mac drivers (as they did until recently). Porting the Linux driver would be a massive amount of effort, even if nVidia was more open.
If you have a slightly older nVidia GPU (1000 series or older), it will work in High Sierra via nVidia's in-house drivers. But you will never be able to upgrade past High Sierra.
If you have a slightly older nVidia GPU (1000 series or older), it will work in High Sierra via nVidia's in-house drivers. But you will never be able to upgrade past High Sierra.