[DjangoReinhardt]

True that.

The answer is also kinda obvious. It started it as a matter of convenience ("I'm too fking bored to type out my long-ass password" or "I have so many passwords, I can't be bothered to remember them all" or "LastPass? What's that?") and has remained so till date. In fact, it will continue to do so until a zero-day exploit appears that can uncover these plaintext passwords, which, judging by current events, doesn't seem too far away