Personally I find that they can, but they don't have to. I can put off Slack/email until I hit a stopping point, or I can wedge it into the middle of a long-running process (start a build/test-cycle/deployment, then go answer Slack). At the very least I can finish the chunk of code/whatever I was working on. Phone calls and in-person talking are a hard interrupt, drop all my context on the floor and get nothing else done.

Those feel more comfortable and they count has hours worked, so it doesn't matter that they are far less time-efficient.