There are many reasons for this. Maybe they're afraid to find out why their spouse is withdrawing or possibly they don't feel they have the power to change or improve the situation. More than likely, it's because their coping skills are based on avoidance. They have learned to avoid emotion, confrontation, or anything that might cause discord.
Sadly, these are often the clients who come to me when it's too late to fix their relationship. Their spouse has been checked out for a long time and they have done nothing to repair the marriage. Usually, their partner has been sleeping in the other bedroom or has even been unfaithful. By the time they do come to see me, the withdrawn spouse is usually very angry because they feel they've tried everything to get through to their partner, but failed. They’ve already moved on emotionally.
Regrettably, many people have very low expectations of marriage and truly believe that it’s normal for their relationship to deteriorate. It’s not. I believe everyone can save their marriage if they are willing to do the work. I believe that you can get your best friend back and truly enjoy each other and have your needs met. The trick is to call a therapist soon than later and not to wait too long.