There is a difference between blocking someone, and being in a line of traffic - even if it's moving quite quick, and even if there is a bit of a gap in front of you.

If I come up behind someone, and I can see there isn't loads of empty road in front of him, then I'm happy to wait behind. Why should he move over? He may well want to go quicker, but knows there is no point nailing it for the 100yds up to the traffic in front.