As others have said, for their comfort. Some members are more comfortable showing skin than others.
I see a lot of “what about Jungkook’s abs?” but Namjoon and Jungkook are different people. Jungkook is very comfortable showing off his abs (that vlive where Hobi was talking about abs and so Jungkook looked into his shirt and was like “I still got it” was so funny) and recently Jimin has been okay with teasing people a little during Serendipty performances too... you can see him smile after showing off a little.
But not all the members are like that. After Seokjin’s abs got exposed videos started popping up of him tucking in his shirt, and while it didn’t do much I’ve noticed Jin fan sites have stopped sharing ab pics. Hoseok also wasn’t comfortable doing the “abs for 10 million Vlive followers” and did aegyo instead.
In general I find a good rule to follow is to enjoy and celebrate what they do (Jungkook seems pretty proud of his abs and enjoys the attention he gets for them, and rightfully so, he worked hard!) and to be respectful to the members that aren’t okay with it.