Because when you saw it you walked 360 degrees away

As a teenager circa 2007, I would troll in World of Warcraft's Trade chat with a variant of this joke.

360 is a full circle, e.g. you'd turn right back toward it.

Au contraire: https://i.imgur.com/PS1Vb6p.gif

Let me help you--that's the joke.

360 degrees away? Why don't you try this and see how far you get. Look at something now turn a full circle (360 degrees) and start walking, become amazed that whatever you were looking at is now closer.

365? Also can’t tell if you’re serious or joking.

The joke is that MS brands its online services as "365", but they have enough outages for people to say things along the lines of "more like Microsoft 360, amiright?"

We use this joke at work all the time, any time a Microsoft service is down we call it “Microsoft 359”. It’s the work equivalent of a dad joke.

Huh, when I saw it first I read it more as: "360 is a full turn, so Slack360 just means same thing as Slack"

This is unintentionally funny

Lol Im meant to joke about the Microsoft product naming and got it wrong which somehow makes it more funny (read the other comments why)