I don't randomly run into colleagues while eating lunch on Slack. I do in the cafeteria.
If I need to casually run something by someone else without raising hackles, I can arrange to bump into them in person. I can't do that in email/Slack.
Text chat is a useful tool, but is not a full replacement for the human experience of walking around other people in 4D.
In your example, if your company were remote-first, the head of marketing would always be on slack and you wouldn't have to wait for the opportunity of inadvertently running into her just to have a conversation. And so would be your colleagues with whom you (apparently) can only interact when you happen to have lunch together. And so on.
You never "raise hackles" by asking something in Slack, because chat is unintrusive by definition (as opposed to walking up to a person uninvited).
Text chat allows people to "overhear" conversations in an unobtrusive way to both the person doing the eavesdropping and the person(s) talking.
In-person requires more active engagement to do the same thing -- you have to go and interrupt that conversation, or awkwardly put yourself into the conversation and "listen." If your entire company is in the #general channel, you will naturally discover things passively eavesdropping that you wouldn't see or hear otherwise in a physical office.