I appreciate that but maybe just signs at the beginning could be changed before they go home? Or maybe that is too simplistic and I'm missing lots of things!
Roadworks have speed limit reminder signs throughout their length, and they're heavy lumps of metal. You'd need to send someone along the motorway carrying enough signs to change them
all, and getting off their van carrying a large lump of metal at regular intervals, each time exposing them and other road users to more risk. For a long stretch of roadworks, changing the speed limit signs before they go home would have to start at lunchtime, and changing them back would start first thing in the morning and finish just in time for lunch.
All of this discussion illustrates neatly that things are often rather more complicated than those of us who are not experts generally realise. As it happens I've just spent some time driving on Italy's motorways, at which point you realise that Britain's approach to road safety is, while not as good as we might like, pretty robust.