Because you are combining the weight into one vehicle. It doesn't matter how much you are carrying it's what it's capable of. Just get a CDL and be done it not that hard and a couple hundred bucks then you can drive whatever you want. Put it this way just because a semi is empty doesn't mean you can drive it because it only weighs 20k and is under 26000 (I'm just guessing). Not all semi trucks have air brakes.