Why doesn't someone nudge you with the professional equivalent of "um, you've got some spinach in your teeth..."? Here are some possibilities:
They are secure in the belief that if they keep their heads down and avoid eye contact, someone else will get the job done. These are the people who never raised their hands above their heads playing volleyball in gym class. ("Here it comes! You get it!" being their rally cry)
These folks notice, but assume someone had to have mentioned it, so they don't want to pile on. They reach the summit, but instead of planting a flag they start looking for the Starbucks the first guy franchised.
These people assume it must the universe giving you a little comeuppance, and, really, who are they to interfere?
Battered Ego Syndrome
These souls see the mistake, but then have a silent argument with themselves that they must be wrong. When the mistake is revealed, they sigh "Oh, I knew that, but..."
Next time you notice a mistake, tell the person. It's nice to be nice, but, nicely or not, I'd rather know when my slip is showing (Hmm, does anyone wear slips any more?), my teeth are besmeared with foreign objects, or my infinitive is split wide open. And you?