"Clearly lost" is in the eye of the beholder.
By the way, being down a queen or even a couple of pieces is irrelevant if my next move checkmates you or forces a stalemate or draw by repetition.
Opponents may be clearly lost positionally or materially but if you don't have enough time on your clock or otherwise can't close the deal in the time you have left then it is YOU who is clearly lost.
That's what people don't get about live chess/blitz games.
They simply expect opponents to just roll over and do the submissive puppy maneuver. There is no such rule requiring your opponents to do so, consequently - if you are truly winning, PROVE IT.
And yes, I know we're talking about blitz games in live chess. In a correspondence game where time controls are measured in days per move, you have nothing cooking, no chance to save the game or salvage a draw and have confidence your opponent will find the right continuation then that's a little different - to resign when you're busted in this instance is considered a sign of respect and courtesy. HOWEVER, once again there is no rule in chess that says a player who is busted must resign.
SO I'm confused as to why we see multiple threads about this issue in the forums nearly every day. Yawn.
Well said! I like the PROVE IT. Don't yell it PROVE IT!

At the risk of going off topic, may I ask: What is the common practice on resigning? This month I have two much better players on this site resign long before I would have expected and in one game before it was really decisively won. Generally, I resign two moves after I decide that there is no hope and no prospect for any hope in a game, but I get the feeling that people expect me to have resigned sooner. Is there some unwritten rule I don't know about?
You can resign whenever you want to resign but it is considered common courtesy to resign as soon as you have a lost position which you know that you yourself could convert to a win (had you been playing the opposite side).