Aside from the distracted, or self indulgent, I think the issue is more pronounced where there's more cars simply because there's correspondingly less time in the driver's day to perform their duties. They (I being one of them) can be pretty impatient, especially with large cumbersome vehicles.
To be clear, if a large vehicle is trying to move to the right lane (or exit lane), it's ludicrous for all involved to prevent that from happening. On the other hand, if it's wanting to move into the passing lane, I may or may not be patient, depending on if this is the first or last in a series of potential traffic interruptions on a given day.
Last year I pulled my trailer through the Seattle and Portland metro areas. I was pleasantly surprised how accommodating the other drivers were as I fumbled from lane to lane trying to stay on route.
