It’s a round-trip latency difference. Standing in the same room, we can get perhaps 100, but certainly 50, responses in an hour from a small group. Video conferencing might be 15% less and that relatively slight difference is already frustrating. In asynchronous written form, it probably takes 3 days to get 100 “turns” and maybe a week or more if there’s timezone “smear”. The written async responses can be higher quality on average, but where reaching consensus in minimal wall clock time is important, face-to-face wins.

Why is Slack “better” than physically mailed correspondence? It’s roughly the same phenomenon, just on an even shorter timescale (including that Slack isn’t better for everything).