There is no mechanical reason not to have curved lines in diamonds - especially virtual ones. I guess the facet cuts have developed over time to make sure that all the light coming from above the gem is reflected back up inside the gem to get the most interesting play of light when you look at it. Curved shapes would have a different effect, and there's nothing stopping you from experimenting with that to see what happens if you have access to a realistic renderer where you can adjust refractive indices and where you get realistic caustic effects.