I wonder if it's just a difference in JP to EN readership or something. A lot of the times in stories like this, when you introduce one really great girl, we'd rather just see the romance play out and have a good comfy story, with any drama being between the main pair and them trying to navigate relationship stuff. But I dunno if it's just that maybe the authors don't have any relationship experience, or expect their readers to have any, so they don't want to write that, so instead they fall back on adding 3rd wheels out of nowhere. But it's always a disappointment to see it happen, so many of these stories would be 10x better without the time spent on a love rival you know is going to lose. Hokkaido Gals, Ayumu, etc. etc. the list of manga that would be 10x better if they just stuck to the main couple and their own drama is endless. But no, always with the unneeded love rival, even if they are MARRIED FFS.