Lol you think they are doing nothing? I'd recommend you try to learn a bit more about their job.

You have safety observers who's job it is to stand around and watch for anything that can hurt someone and stop it from doing so. You have the site engineer who's job it is to stand around and make adjustments as needed to the design and documentation. You have the job site supervisor who's job it is to manage the people on site and make sure that the rules and regulations are followed. You have apprentices who's job it is to watch and learn what they are expected to do and lend a hand when told to do so under the watchful eyes of more senior workers. You have specialists who's job it is to be on standby until it is time to do their specific piece of work. You would not expect excavation specialists to be able to do plumbing, or electrical work, or gas pipes?

There are specializations that each worker has. Just as you would not ask a roofer to wire up a house or a framer to plumb it, so to there are specialization in these types of jobs as well.

It is cheaper and quicker to keep them on site so that when their specific part of the work is needed it can be done right then and not have to call them and stop everything for an hour or two while they get there.