BT5 supposedly got the tattoos as a "friendship" thing, but I don't believe that's the real reason. After the outrage at JK's tattoos, I think the label has told them all to get the 7 tattoo and claim it's a friendship thing, to normalize tattoo culture. SK has very intense views on tattoos and they aren't typically viewed in a positive way.
That being said, I think the label also knows that other groups and people will copy BT5, and do similar things with tattoos. In this case, G0T7 are doing the 7 tattoo, probably cause the number is in their group name, but also to follow BT5.
What weirds me out is tattoos are personal and permanent. JK was the one with a large quantity of tattoos, then JM started getting more, and now it's gotta be the whole group. I don't care for JK's tattoos aesthetically, but he stood out in a really cool and bold way.
All the members except SG have shown their 7 tattoo. Makes me wonder if he's gotten it at all.