Wouldn’t a couple be colluding in divorce if they just agreed to divorce?

No, no more so than a couple who separate in preparation for a divorce have deserted each.

Collusion requires some sort of fraud on the court. The mere agreement to divorce, absent a lie or a fraud, is not collusion, and is in fact a requisite step in setting a separation date important for the classification of marital assets. Of course, the mere agreement to divorce was not, traditionally, an adequate grounds for divorce prior to no-fault and liberalization of divorce.