To be honest, I always viewed this game as very laid back and relaxed when playing it. The kind of game that didn't really need a plot, or might even be an "open world" plot where there isn't really a great desire to achieve any main goal, but have there be some kind of reward (in this case, capturing girls) for players who do decide to progress the "plot". It would take a hell of a lot of effort to make a convincing story, so I think keeping it the way it is would be the best course of action. I would prefer what we have now to a story that falls flat.
Edit: I just remembered a pretty good example, in my opinion. Eroico by Kyrieru. It's a game that doesn't really have a plot, but is still thoroughly enjoyable due to the gameplay and animations. Sure, a story might be nice, but if it wasn't as good or better than the gameplay, it would detract from the game as a whole.