Because you're moving from drive to drive, not drive to array. You should see how much free space each drive has available. You need to select a small enough portion of the source drive to fit within the space available on the target drive.

I see... it's almost like it's ignoring the data itself, and just blinding moving the entire thing at once. It's not taking "folder a" and moving it's contents to "disk 1, disk 2, disk 3".. it's literally taking ALL of "folder a" and moving it from "drive 5" to "drive 8" and not moving the contents around to other drives.

I'm sure it's been suggested, but if not; perhaps a new future version could be more granular like that and take "all selected drives" and spread out the contents you select into them, rather than root folder to one drive.