firstly, "doing useful work" for these trucks includes sitting still while being filled with spoil, not just driving around looking all fancy

it's not about simply "coordinating". Logistics requires that you have the capacity to absorb unforseen issues and delays. When you have a system with 100% utilisation, the slightest delay at one point ripples out upsetting the balance of everything else and suddenly everything grinds to a halt.

For a more techy example, it is a bad idea to have a server running at 100% CPU usage. If that is your "normal" state, any change in conditions for the worse (more customers want to buy your product because its a weekend) results in degraded experience or total failure for ALL the customers.

And construction (and large machines) just LOVE to throw delays at you.

I'm sure they have people who's job is coordinating. In fact to coordinate such a photo op would be an almighty feat in itself. It's just if those people are in any way good at planning (their job) then they would leave slack in the system, the exact amount is the real trick to avoid waste, but it's never 100% utilisation.