What’s the reason for this?
Some of it is depending on how high up you are something can bounce pretty far, possibly outside the barricaded area. The term used is ALARP- As Low As Reasonably Practicable. The trade off is, when someone's working really high up you can make everyone else stay inside but this wouldn't really be productive so you barricade off directly under where the work is going to take place and where you could anticipate something bouncing too, then do everything to still prevent dropping something. I mentioned with the tools, everything needs to be secured, so for example if I'm removing a CCTV camera, I need to attach some form of attachment to the camera before un-bolting it. Of course it's still not possible to secure the nuts or bolts holding the equipment in place so move slowly, pay close attention to every move made. If something does get dropped, there is more paperwork to do and everyone in the company is notified. They calculate what the potential harm to someone is [1].
[1] https://www.preventdrops.com/safety-regulations/understandin...
In large-scale construction, safety needs to be built into every aspect of planning and execution of the project. That can add large costs.
https://www.latimes.com/sports/story/2020-06-12/attorney-for...
The other aspect of it is general workplace safety regulations which have expanded over the years as well. When budgeting a project, you need to consider both, however. And workers also benefit from both. For example, the restriction on the use of asbestos in building insulation was brought in to reduce the health hazards of living or working in such a building but the construction workers handling the asbestos are clearly the most at-risk group which benefits from the regulation.
Source: I worked on a deployed aircraft carrier, one of the most dangerous workplaces in the world.