Why does that bother you?
Because it makes me think he doesn't understand it's over, which is an offensive level of stupidity.
Let me start with a blunt assessment, your reason is really rough ridin' dumb, and isn't actually a reason, I'm 100% sure you know it too. A reason is something that can be presented to Sanders that would make him actually think about quitting. "You're behind in delegates in a two person race," is just as stupid as "You're in 5th place in delegates awarded after three states, and no other candidate has been able to over come that."
Is this the first presidential primary you have followed? There hasn't been a non-incumbent primary in our lifetime in which all of the nominees have cleared the field at this point in the race, especially for a candidate that has only spent three weeks leading. Bernie dropping out now literally serves no one well at all, especially not Joe Biden.
You are letting Trump erode away the logic and common sense parts of your brain.