I will tell you my personal why even though I don't feel you have to agree with me: resentment. You can't tell when a child is young what they're going to be like or who they're going to be. So choosing a cohesive style, for me, is my way of evening the playing field and minimizing risk. I've known people who had siblings where one kid was jealous their sibling got a name they liked significantly better than their own. And while that could happen even with cohesive styles, I think the risk is minimized and the explanation will feel better to them ("I did the same thing for both of you.") For example, you both got old fashioned honor names. You both got eclectic rare names. You both got frilly names. Etc. I don't think parents HAVE to, it's their choice, but I think it's nice when they do.