Why do slow lorries try to overtake slow lorries on the motorways?

Thats right they are governed to 56mph and each truck varies by 2-3mph! It's a soul destroying thankless task. You set the cruise on the motorway and p anything slower than you! Absolutely mind-numbing! We don't like to be a rolling road-block but if you slow down to stay behind, it could take 10 miles to get that momentum again!