I forgot about the ritual significance, as for the need for private for ownership, that is a more philosophical question that can be stretched forever (can one truly own when both object and owners are finite mortals? And other such lines of inquiry), personally I believe it is vital as a prequsite for the rise of the concept of the individual and what it entails. In this context : that the object of ownership cannot be taken without permission or negotiationed recompense, as otherwise it would be not qualify as owned. More so since Ymaryn society is one of an active and powerful central state.
But there are other reasons that I am too sleepy to state with much cohesion.
Nevertheless, you make an excellent point. And organised leasing would explain how a form of contract or deed can be developed.