i like to think of it like this: they create a new universe for their story and when they enter it obviously they are at first drawn to the flashy couple with all the “perfect” attributes like “strong alpha” and “tormended omega” but while they continue to flesh out their story they stumble across the people that are actually unique, even if they are ordinary or dont fit into those perfect categories, but at this point its too late to change the story entirely so they have to stick to the main couple and as they continue writing they lose more and more interest in that couple so the plot also suffers …

somebody needs to study the phenomenon of authors writing their main couple super generic and boring (like here for example with the classic strong alpha x helpless/kinda dumb omega) but they accidentally create a really great setting for an actual interesting (side) couple that have complicated dynamics that make you want to explore their individual characters … its happened so many times honestly (liveta for example with van and ian, or the royal bridegroom and so many more that i cant think of right now)