Because these days, most of them don't. For many couples the problem begins with a mistaken notion of what marriage is exactly. Jewish wisdom assesses the highest priority to clarifying life goals. Life goals also create a context for living that enables couples to put other matters into perspective. Couples may argue over a stray toothpaste cap, the style of a new couch or whose turn it is to get up with the baby, but no matter how heated these run-ins become, they should never destroy a marriage.
Marriages dissolve when two lives are pointed in different directions.
Conflicts over the color of a new kitchen can generally be resolved, but conflicts in direction often cannot. Couples rarely break up over clashes in taste, but they do break up over whose career comes first when the two conflict. Couples will break up over whether to give priority to career or family, over whether or not to have children, over the education of their children and over which religion or how much of it to have in the home. These, and other issues like them, are anything but trivial. These are life goal issues.