Why don't you have the new Carerra GT? Maybe you don't have the money? Star wars universe is in a lot of ways like our own. Wealth considerations do apply to all the species we see. The trade federation are obviously very wealthy to have such a strong and sofisticated army, but they are not unlimited of course. Their budget, and their training time are limited. Plus they have to make decisions on allocation aswell. Cloning Grevious's or making more like him would obviously limit the rest of their army. Maybe far less droids, or less star ships. Maybe they don't feel the benifit out weighs the cost.