Well, the extra weight and cost of the equipment are two points, small in comparison to the weight of the trucks. The accountants (bean counters) for the car manufacturers are always trying to keep car manufacturing costs to a minimum as saving a penny on 50,000 cars makes a difference.
Not only that, the brake shoes and pads are much more expensive on trucks due to the sizes involved - stopping 30, 40 or 60 plus tonnes takes some doing.
So, jake brakes, or eddy current braking systems as used on coaches mean that the running costs of the trucks and coaches are kept down. Also the engine braking systems are common as well.
Also, the drivers tend to be watching the road conditions and slowing gently earlier than "some" car drivers to minimize their braking anyway.