Some of that may be due to the owner transporting it from job to job instead of storing it in their own yard.

Nonetheless, there's a lot of idle people/equipment in construction because things have to be done in the right order and sometimes it's more economical to over-provision than risk delaying the whole project because something wasn't available when it was needed.

Those hundreds per day will partly pay for all the sitting around unused in the hiring company's yard! You can see this in the pricing which is usually a lower rate if you hire it for a longer period.