Because the ancestral ability adjustments are just smoke and mirrors. Because of the layered way you pick ability bonuses and the number of floating bonuses you get, it’s possible to put 18 in your class’s primary stat, a 14 in your class’s two secondary stats, and a 12 in any other stat of your choice, regardless of which ancestry and class you pick. Functionally, PF2 might as well just have the choice of an 18, 14, 14, 12, 10, 10 array or an 18, 14, 12, 12, 12, 10 array. Try it, pick an ancestry and class combo you think wouldn’t work well together, follow the steps for character generation, and see if you can get one of those two arrays (you can).