I don't find that surprising. Most of the communications I have at work are with a specific person, not with a whole channel. It's not because I don't want other people to know what I'm saying to someone, it's that I don't want to bother them with stuff that isn't relevant to them.
How do you know you bother them? Your comment makes me think to daily meetings in SCRUM. Sure most of the stuff people are working on or that blocks them is not relevant to everyone else, but we do it because it's good for the whole team to share and stay up-to-date on what is going on. As a tech lead/manager, I apply the same logic for my communication. I use DMs only for stuff that shouldn't be discussed publicly.
