Well until the issue is resolved you can't know how long you've been down for, so you can't actually update the uptime.

Why not? It could be updated second by second automatically if they wanted to.

Probably not a priority though.

Building out the infrastructure to automatically give real-time updates to your uptime figure sounds like a terrible use of company resources. Who knows how many person hours to spend on implementing and maintaining a feature that would remove maybe a few minutes of manual work from the incident post-mortem checklist, just for the sake of delighting people who need something else to look at for a workplace distraction now that Slack is down.

Well, now the outage is marked as resolved. And the uptime is still "100%".