The only thing I will give you to chew on is why do we ground the service at the service disconnect? It is already grounded at the transformer.
Basically, because the origination of the EGC needs to be grounded, and a service drop does not include an EGC.
A non-EGC run to a detached building would originate an EGC, so that must be grounded, too, much like the service drop.
A run to a detached building that includes EGC (4 wire for 120/240 split phase) better protects other metallics (twisted pair, coax, door bell, ethernet ... a list that was not so large just a couple decades ago) between the buildings by not having any current carrying conductors grounded at both ends. The EGC still needs to be grounded (again) at the detached building, right?
So what if we did have separate EGC on a service drop? We'd ground that at the service entrance or disconnect in the served building. What if the MV system neutral connection back to the substation breaks (e.g. lightning or or falling tree branch takes it out)? The return then is ground ... any ground anywhere. By having grounding near the building, shared with incoming metallics, at least we can significantly reduce the step distance experienced at the building, and thus the MV potential being radiated out from the transformer location (even if it's a pad).
At least with the service drop not having its own EGC, any circulating currents have to involve the resistance of ground.