Because our aim is to be sustainable and its exercise is placed among myriad options. As the word sustain itself means to hold from below the purpose of this rootedness is permanence. As permanence is not a substance we can only hold it as an ideal. It is the same for “zero entropy”, the name that would achieve permanence with matter.

Any integrated agricultural system is a practice. Practice still understood as an ongoing deed that pursues an ideal. And any practice must be affordable to its Patrons.

From http://www.i-sis.org.uk/DreamFarm2.php

Above  is an exemplary "zero" entropy integrated agricultural system.

Below are the features of sustainable agriculture.

"Sustainable agriculture" was addressed by Congress in the 1990 "Farm Bill" [Food, Agriculture, Conservation, and Trade Act of 1990 (FACTA), Public Law 101-624, Title XVI, Subtitle A, Section 1603 (Government Printing Office, Washington, DC, 1990) NAL Call # KF1692.A31 1990]. Under that law, "the term sustainable agriculture means an integrated system of plant and animal production practices having a site-specific application that will, over the long term:

• satisfy human food and fiber needs;
• enhance environmental quality and the natural resource base upon which the agricultural economy depends;
• make the most efficient use of nonrenewable resources and on-farm resources and integrate, where appropriate, natural biological cycles and controls;
• sustain the economic viability of farm operations; and
• enhance the quality of life for farmers and society as a whole."

From http://www.nal.usda.gov/afsic/pubs/terms/srb9902.shtml#top

As you can see from the diagram above, recycling every farm calorie- except that lost as heat, for which there is no capture- is an elaborate and capital intensive proposition. It is why sustainable production transcends organic production even as the latter aspires to the same goals as outlined in the bullet points above; but, does not practically mandate ideal zero entropy food production in its organic production bylaws.