As I've read in other forums the Rivian EAVs are targeting the outdoor adventure demographic than the worker that needs to haul around lumber and other large construction materials (e.g. Atlis XT or Workhorse pickups).
I've also read people (speculating) it might (also) have to do with the structural strength of the cab as a solid window provides more rigidity than one that opens. (I assume the implied here is in case of a rollover.) Seems a little bit of stretch to me but I'm not an automotive engineer. I suspect it has more to do with the target demographic, though admittingly that also seems like a weak reason. Who knows, that might be one of the "small tweaks" Rivian makes for the production model.