Slack was/is pretty terrible too. Having every workspace require a new user is the pinnacle of idiocy. So annoying and even worse if you have different emails for different workspaces.
There are plenty of reasons to do this, all of which have to do with, say, privacy, and Slack has managed their way into making it dramatically less annoying.
1) They send magic links. Pretty easy.
2) They make all known workspaces you've logged into before discoverable and allow for a one-click "add to desktop Slack" option, which makes dealing with the whole "different users" issue. And to the extent that I use different emails for different workspaces, Slack accommodates that and allows me to do so within the same desktop instance, so not really sure what the concern is there.
That's true, but it doesn't change the user story going from "I click a link, I join the workspace" to "I click a link, I fill out yet another registration form, decide which email to use, add another password to my password safe, then join the workspace". Minor differences but friction does matter.
To add onto this, you also have no cross platform user consistency, so if I DM a user, if I want to search my DMs there, I might have to search my DMs in 10+ servers for the specific message I want. There are a million other problems with this model but this example is definitely one that I frequently ran into before people I knew switched to discord
