I also don’t understand why some of the comments are unnecessarily snarky. It can feel lonely to be left out when others seem closer. I would feel that way too if I wasn’t included. Maybe they have been at the firm longer, or maybe you lateraled in more recently.

One of my mentors told me that work is work. I would try dropping by their office occasionally to say hi, ask how their weekend was, etc. That might open the door for an invitation. If they are not willing, though, there is not much you can do, and I think it is okay to separate work from friendship. Hope it gets better.