When a semi-truck tries to overtake another truck with a minimal speed difference, blocking all lanes in the process, it is known as "Elephant Racing" -- extracted from the german word Elefantenrennen.
Recently, a r/Trucking Reddit poster asked, "...why do two semis sometimes pace each other with one in the left lane, blocking the passing line and continuing in that manner for long stretches?"
We've all been in the situation as general motorists: cruising between 70 and 80 mph when you find yourself quickly approach the rear of a big rig. Cut the cruise control, brake and slow the traffic behind you. You may even release a few cuss words under your breath (yeah, we understand!).
In the opinion of the Reddit user Hard_at_it, "It happens because expected courtesies are just forgotten in this generation of drivers. Some drivers feel its an injustice that they are being passed and make it more difficult. A true, professional driver would back off slightly to expedite the overtake."
He went on to say, "My truck is governed at 62 mph; what really grinds my gears is when me or my trainee will move over on an entrance ramp to allow a smooth merge, and then that driver will hammer down and pass us on the right instead of allowing us to return to the right lane."
Another user, kellanium, said "The overwhelming majority of companies govern their trucks. (Mine) does 64; Crete does 63; Prime governs at 60. The issue is that due to a number of factors (tire wear, drivetrain configuration, weight of load) you can have the left hand driver going 63.5 and the right going 63.3. Common sense and courtesy would suggest that the right hand driver back off on the cruise.
Ultimately, truck drivers and general motorists alike do not lose (or gain) much time using this type of approach. It is more of an annoyance and nuisance than anything else.
The Reddit user tcfjr had an interesting comment: "It's especially bad on I-5 in California, where the speed limit on rural sections I-5 is 70 mph (112 kph) for cars, but only 55 mph (88 kph) for big trucks. (Everybody drives 5-10 mph over the posted limit, but that still results in a 15 mph (25 kph) speed difference.)