I used to think 'I didn't see him' was a pretty bullshit excuse and it really just meant 'I wasn't paying attention', but recently I found that sometimes you really can't see them.
A few months ago I think, I was sitting at a stoplight waiting to turn left. There was a pickup coming my way, but the street was otherwise empty. As the truck approached the intersection, I started creeping into the intersection to turn behind him, and when he passed I started turning- and then hit the brakes hard enough to remind me the ABS doesn't work. Slightly behind the truck in the far lane was a guy on a sportbike [that probably now needed to change his pants] that was completely unseen from my vantage point. Even though I was barely moving, he was probably doing about 50, and if I wasn't paying attention I could've ruined his day pretty quickly.
So maybe they can't see you or maybe they aren't paying attention or maybe they don't care- that just means you have to pay attention for them and anticipate the unexpected whenever you go riding.