What you are describing is how people use the left lane during light traffic; a temporary labe to overtake one or several vehicles. During heavier traffic, most drivers don’t do that. They use the left lane as a, “fast lane,” rather than entering and exiting the middle lane repeatedly (which, strictly speaking, isn’t how you’re supposed to use that lane, but it’s how the vast majority of drivers use it). That’s usually when some asshole decides the, “fast lane,” isn’t fast enough and starts tailgating, forcing people to merge into the already crowded middle lane and slowing traffic even further.