Question for truck drivers about 3 lane highways

I drive I35 between Austin and Ft Worth pretty much weekly. I drive using my cruise control set at the speed limit (the majority of the trip is 75mph), and stay in whatever right-most lane I can, only using the middle and fast lane for passing. I cannot count the number of 18 wheelers I pass on the right that are in the middle lane doing anywhere from 5-10 mph under the limit planted in the middle lane. It’s dozens and dozens every trip, and they always have a clump of cars behind them trying to get around them on the left and right. What’s up with that? When they see a stream of cars and other big rigs passing them on the right, why don’t they move over? Do the good truckers call them on the radio and tell them to move over?

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