Comment by crote

This used to be the case, but at some point in the last few years Reddit started suggesting usernames like that to new human users. It'll generate stuff like "Fine_Ad6357", "Unhappy-Benefit8521", and "SuccessfulShape2454".

I believe they started doing this because Reddit got really popular and all the normal usernames were eventually taken. I wouldn't be surprised if they implemented this after noticing that a significant number of signup attempts were aborted due to multiple attempted usernames already being taken.