There’s almost always more than one reason for something. “Stay away from the river: you’ll get wet and catch your death of cold; you’ll drown; it’s full of hungry piranhas”. Any and all of those are possibilities. But at least there’s fun to be had in going there anyway. Then there are lorries: big things; slowish; difficult to see around; lots and lots of kinetic energy. No fun at all to be had in going there. But lots of reasons to avoid the things.
The most common training mantra regarding lorries (and other suchwhich) and the overtaking thereof, is to hang back, clear of the vehicle, until your exit beyond is clear: i.e. do not sit up the back of the slow car overtaking the slower lorry, but wait until it’s crawled past and (hopefully) pulled back left. Reasons are, I hope, bleedin’ obvious – Big vehicles have Big blind spots. And you’re on something very small and vulnerable, should the Big decide to go elsewhere than it’s current trajectory. So, keep clear until you can get past in one smooth, fast manoeuvre (both wheels on the ground, by preference).
Further to the above, if you’re on a single carriageway road, you’re minimising your exposure time to oncoming vehicles and, on any road, you are giving yourself the greatest number of degrees of freedom of movement, to go forward, backwards or sideways depending on need.