The production cost of a drama is significantly higher and since dramas have better purchasing returns from how the content is distributed, there is more focus on the story/character development (unless we are talking animated drama). Comics/Novels have low production cost and significantly more ptions for readers, but nowhere near as much purchasing returns, so art tends to be the focus since the cover art is often a deciding factor for a reader to consider reading. Granted, that doesn't mean there aren't creators who don't do a good job with the storyline, because there are, it's just sadly the exception rather than the standard, especially in popular genres.
NTR while common in quite a number of harem and sm.ut stories and some romance is not necessary for harem (unless the harem is at the point where the lead engages in intimacy with more than one partner, but most would not see it as NTR if all the other partners are accepting of the arrangment). It's a plot device to create tension and/or evoke mixed reactions in the audience (NTR is the act of a character deliberately cheating on their significant other with someone they aren't in a romantic relationship with).