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.

The rent/buy calculus can be incredibly shunted towards "buy" once you're using it even somewhat. It's even more so when you realize that labor costs is the main issue, and not having a skilled worker waiting around for an item to be delivered (by another slightly less skilled worker, perhaps) is a huge savings in its own part. Five guys on a site might have 15 machines total; even though they obviously can't use all 15 at the same time.