Having worked all my life on roadworks in one capacity or another, perhaps I can give a few insights.
Whilst 'nothing is happening' there could actually be lots going on. Concrete is designed on its '28 day strength', because it takes time to cure and come up to strength. If you are building a new concrete barrier, you can't do anything else to it for that period. Thats why you do long stretches, so you have always got somewhere you can work. You could argue 'do a bit at a time', but putting the traffic management(TM) in and out costs money and it is freaking dangerous. I know at least 3 guys who have lost their lives doing this. Even when the TM is out, everyone is in such a hurry, it is freaking scary being on the works side of the cones...go and try it sometime! Nowadays we have the temporary metal barriers which help. I remember before they came in (and before we had 50mph limits) one guy who just wasn't paying attention, swerved to miss the car that had slowed in front of him. He swerved to his right, through the cones, over a pile of sand which launched him in to two guys working. One is (as far as I know) still in a wheelchair and the other lost his life. Other works eg white lining, are very automated now, so you need long stretches to make it economic for them to come and do the work, but overall the main reason is our safety from all the speed demons who are trying to save a few minutes on the way home or something. Like the signs say 'Lets all get home safely'.