Why doesn't my suit jacket ever look good when buttoned?

I can't figure it out. The one jacket I had tailored, they actually moved the buttons in order for the jacket to fully close and not reveal the bottom of my dress shirt (you know that little triangle space?) But I can't imagine everyone is having their button adjusted for that.

I don't have a particularly big stomach. I buy decent suits. why won't it button and fully cover my dress shirt?