Because bad things happen.
There are driving laws all over the world that are aimed at keeping people safe and preventing accidents. Traffic on highways and multi-lane roadways usually has a pattern where fast cars stay in the left and slower cars stay on the right. Passing is supposed to be undertaken to the left. This avoids confusion and helps create a smoother flow of traffic as opposed to everybody just driving as they please.
This is an extreme example of why you don’t pass on the right and should never even consider passing on the shoulder, especially when a tractor trailer is involved.
Instead of just hitting the brakes as the tractor trailer moved into the right lane, then passing in the left, the driver decided to pass on the shoulder. That ended up being a very bad idea as the driver went off the road and into the ditch before hitting a big piece of earth and coming to a halt.
That’s why you don’t pass on the shoulder, especially when it’s a truck, kids!