@Gladiator02 Well, it seems the REAL reason is because they like spending time together. The pretend reason is more about honoring their parents. Even if they really did hate each other, they probably feel obligated to honor their parents' wishes, and looking like they hate each other, even if they are going through with the marriage would probably make them look childish and embarrass their parents. Honoring your parents is a key tenet in Japanese culture...even more so if you are from a traditional and rich family.
@Sirona They are both rich. It's a pretty common trope in manga and anime that rich parents from traditional families arrange marriages for their kids (that kind of stuff does still happen in Japan, but usually the couples are adults when the marriages are arranged and have some input on the arrangements.) Their parents are probably friends, or have a crucial business relationship.