they went to the parts bin for the most heavy duty overbuilt design availble: the Landcruiser hubs are the same way large 150 mm bolt circle (which reduces the load on the studs thus less required).

By the way if you have trouble finding 18" aluminum wheels search for a 2006 Landcruiser fitment.
Any wheel that fits the 150 mm Landcruiser will fit the Tundra and you will have about a dozen manufactures making them instead of just one for the Tundra