Years of socialization training stemming from childhood to gauge whether it's really worth interrupting people. I do enjoy the opportunity to do remote work, but even in the office, I'm finding an unstated expectation that I should always have a second pair of eyes on my slack notifications. It's not really consistent to praise the elusive "in the zone" state where developers get their most productive work done, and simultaneously expect that they should break what they're doing to bucket the importance of every little IM. The need to walk over to another person when you have something important to say, and use your human senses to determine whether this person should really be distracted for what you think is important at the moment puts the onus and effort of distraction on the asker where I think it best belongs.