That's one of the trends of this era. "Coupling" earlier, and the relationship being the focus. I think in a lot of cases, it's easier to write natural tension when an "easy" conflict is unresolved (will they or won't they). If everything stems from that sole issue, you have an available starting point for a multitude of subplots and arcs. If they're in a relationship, that "easy" starting point is gone, and you have to write quite differently. Creating tension in a happy couple is harder to do without it feeling "forced" or "artificial". Neither way is bad, but prolific readers seem to be burning out on endless waits. A lot of the top performers, that aren't brand new, include the continuation past a confession, and do it well.
I think there's also something to be said for the difference between reading 50 chapters for a relationship to bloom, and that particular tension being dragged on for 300. That kind of longevity is HARD to do successfully. I think of Komi, which had SO much else going on, and KanOka, which (in my, and many vocal others opinions) has become a bland interminable cycle; very different works, with very different "feels".
Well this ended up being longer than I intended.