Because its female mangaka writing yaoi for female market
at first i didnt get what u were talking abt but i went back and damn... they rly stole the tops nipples
This was, likely still is in some areas, very common for any form of smut. Drawing things only when needed. Part of it was due to censorships, others really just laziness or not wanting to draw attention away from other things. Similar to how artists don't always draw a dick dangling about, because it's not a sexual scene or one that'll turn into one, hence they don't draw one in that panel. ╮(￣▽￣)╭
But yeah, it was very common in old mangas to just draw nips for the uke, simply because the top didn't need them for any sort of scene. All readers have made fun of that since ancient times tbh. 'Stolen nipples' was basically a running gag from way back when. XD
Ok look, I have complaint. Why Taniya got huge perky nipples but none for Irobe. I feel like lots of BL (female) mangaka tend to make the uke too feminine like....can't you just draw both of them equally manly?