V/max on the BAB of trucks

I didn’t get a bad start today, when I was overtaken by a truck at the BAB in a 100 zone at 120 km/h… Well, that was such a towing tractor for other trucks, suppose the things have a good performance. However, it is striking that hardly any truck drives only 80 km/h. I thought they had been slowed down to this 80 km/h for some time? Why does the majority drive easily 90 km/h, 100 km/h or even more (and watch TV)?