Why can't truckers slow down their large vehicles before they enter a city, instead of using loud jake brakes?

truck drivers, like most drivers that are not senior citizens, like to go as fast as possible as long as possible. and for truckers it is more important, their jobs are on making good time. the sooner they finish a run the sooner they get paid and the sooner they can start their next run. Dollars and cents. Lastly using transmission to slow down revs the engine up and burns more fuel. now coasting long before town would use the least fuel, but that would take longer. So the most econimical way is too use those loud brakes.