It really doesn't cost all that much unless you're cherry picking examples. I divorced with my first wife and we did it amicably and it cost next to nothing. Really bad divorces are not the standard and, even when they do become nasty, it's usually one of the lawyers at fault for instigating.
If, by "cost", you're factoring in division of assets, consider this. Most couples marry early on and succeed as they grow together. Most divorces end up with some sort of financial settlement that is *perceived* to be in the woman's favor. Why shouldn't she get half of what you both built? Even if you did all the work and your spouse was made to sit at home raising the kids, is that deserving any less than half? Those who marry into rich usually do so with a prenup, which is fair. If you're going to pluck someone out of middle class and give them a wealthy lifestyle, it's not fair to dump them back where they came from with nothing more than what they came in with should your relationship end.
Choosing a partner for life has many advantages, not just socially. You spouse is your best friend, the one who ALWAYS has your back, your confidante, your biggest supporter in your efforts and endeavors, someone who will be there with you every step of the way. If you both work, it brings in more resources to enjoy the finer things in life together. It makes a more stable and fulfilling family environment to have two parents when children are involved.
Sometimes people change over the course of time. Sometimes these changes are overwhelming to the other and result in irreconcilable differences. In our modern state of self-empowerment, it often makes the decision process of splitting, when things aren't going well, more easily attainable whereas, in the last century, more people stayed together for the sake of values and convenience and not always for love. There is no crystal ball method of foreseeing how a person will be in 10 years, 20 years, etc. There is no surefire way to divine whether your relationship will, indeed, last forever. Yes, there are ways to improve the odds by using your head but nothing is certain in life and that's what makes life what it is.
Some people remain single their whole lives for various reasons. Some have commitment issues. Some have an incessant need to keep testing the waters to make sure they "still got it" (probably the biggest group of losers on the planet). Some just enjoy being single for the perceived "freedom". Some simply have not found "the one". No one loves being single all the time.
Whatever the reasons are, there is no one-size-fits-all solution. Humans are as diverse as snowflakes.