Comment by shadowfiend

The actual reason here, implied but not stated outright in that one, is that Discord being a public platform, having only numbers to discriminate between users makes it extra-trivial to impersonate someone else. Obviously you can still do some of this with unique usernames (you see slight misspellings, adding harder-to-see characters like periods, etc, as strategies), but these are more complex to execute on at scale and easier to block once and reduce the impact, vs being able to use ~arbitrarily many post-username numbers.