The nature of IRC is to "never remember". Because of this, if you leave IRC, the nickname you used is now open for anyone else to take, and since the server forgets everything once they've passed, it won't recognize a "thief" when it sees one. In the early days of IRC, and on some IRC networks today, people argued that nicknames were "common property" and that no one could claim ownership of one. Thankfully, that mindset has changed, and today we have an IRC service called "NickServ" that registers and protect nicknames.