Why do we need the complex contract to signify that two people love each other? Why not just start living together?
Does being married to the same person for your whole life even make sense?
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I'm going to answer this in terms of the issue you are currently worried about, asset-splitting during divorce. The general idea here is that, in a marriage, both people are taking their partner and the relationship in mind while making decisions about their own lives. Often, this entails financial sacrifices.
As long as the couple happily married, these trade-offs should be worth it; both partners benefit from the relationship. In my own marriage, my husband would probably earn 2-3 times as much money if he weren't married to me, because he has moved multiple times for my career, and we will never live in the best cities for his field. But it's worth it to him, because he loves me and enjoys being married to me. Likewise, there are jobs that I haven't applied for because they would limit my husband's career path too much; but it's worth it to me, because I love him and enjoy being married to him.
These benefits aren't always symmetrical, though. A parent who stays home with the children is the most obvious example, but there are many others. One partner may turn down a great job opportunity because it would require moving away from the other partner's job. Or agree to live in a smaller town, where career opportunities are more limited, in order to be near the other partner's family. Or turn down a job that requires lots of travel, because it would disrupt their relationship. Or go part-time because the other partner is a high-earner, and it makes since for the lower-earning partner to focus on running the home life so that their free time together can be as pleasant as possible. There are endless permutations.
So, when dividing assets, the assumption is that both partners have been prioritizing the overall economic goals of the family, and therefore the assets of the family should be mutually shared, regardless of whose name was on the paycheck/bank account.
Obviously, this assumption doesn't hold for every couple. If you think that getting married will not change your career decisions at all, or those of your girlfriend, maybe you shouldn't get married. But don't be surprised if that's not the model of partnership your girlfriend is looking for.