It's the immense saturation the market is going through right now. Before, there were so little groups around that each of them could pick and choose the styles they wanted to define their group. Now however, being stylistic or rather, being innovative is difficult because almost everything has been done before or, is currently being done.
Also, these days, and you can definitely see this with BGs and GGs, the music has been shifted to secondary. One can argue that, that's always been the case with Kpop. However, before, you can tell that a group's primary focus was to get a hit or keep getting hits. Therefore, the groups would try to different concepts and styles to keep producing the hits, which can be seen with SNSD, T-ara, Big Bang and Beast, to name a few. Fandom build up, was secondary. Now that fandom build up is brought to primary focus, companies will just give their groups music they know fans will appreciate or, are comfortable listening to occasionally. They won't try to experiment, which is why we have groups, like the ones you listed, having interchangeable music. Or even groups like TWICE, BP and GFriend keeping their current music, regardless whether it sounds similar to their previous releases, because they don't want to turn fans away.
I have to stress that this does not mean the music is bad. It just means that nobody is doing anything different because I mean why should they? Hits are not the main goal anymore. The main goal is for you to stan these members and spend money on them.