First, most of us hope for monogamous marriages. Part of being in a monogamous marriage includes forsaking all others. So the idea of "looking for someone better" in terms of dating doesn't really help much. It doesn't prepare you to love one person, regardless of their flaws, regardless of how you fail each other (of course, excluding huge sins of adultery or abuse), regardless of whether or not you constantly have butterflies in your stomach. Dressing up for every single encounter doesn't prepare you for each other when you are sick, tired, grouchy, when your hair is a mess and all you want to do is wear sweats, curl up in a corner and die. Constantly covering up important values because you're so concerned with impressing the other person doesn't prepare you for those much needed talks on finances, children, in-laws, and sex. Constant dating may give you something to do. It won't bring you intimacy.
Second, because we're told to constantly date, yet not get married until we've saved for retirement (HA HA HA!!!! In this economy), I've seen so many girl friends get caught up in a cycle of serious boyfriend after serious boyfriend. They don't even know who they are by themselves. Rather than think honestly about what they would want from a husband as they would about a career or educational opportunity, they willingly let others sweep them off their feet and define their worth by whomever is on their arm. I mean, it's one thing to date because you think there is something deep but it doesn't work. It's quite another to have had a serious boyfriend every year from before you even started your college applications. If you're that insecure about yourself, how can you trust your ability to truly love someone (for better or for worse) until one of you dies?
Honestly, if you want a lifelong companion, that involves carefully considering the traits you would want in a spouse. That does not include looks, the size of their bank account, or the ability to knock your socks off with every single kiss. That includes traits of integrity and character, a sharing of values, compatible personalities, and a willingness to fight for each other so long as God wills you together. And they usually come when you're not seeking. People are gifts, not objects. Perhaps more marriages would be successful if people realized this.