Because they are circular. Like the wheel. Like the planet. Like good relationships. Like patterns in the weather, in the oceans. Like the rain falls, absorbed by the plants and soil, only to be released again as humidity, to rise up into the clouds, condense, and fall again onto upraised leaves, and rich earth receiving it again for perhaps the millionth time in an almost-infinite cycle that will repeat until the planet and galaxy collapse into their next new beginning,
Reciprocity works. Sustainable, in a process, in a relationship, in a market, in anything, means that it can continue. Sustainable is just another way of saying 'it works.'
Everyone wants to put their effort into processes that give back productive results.
The Sun pours energy all over the earth every day. Water falls out of the sky everywhere. If we can take these abundant resources, costing only what effort it takes to collect them, only needing to add readily available seeds and maybe unadulterated soil, and reasonably easily produce the most valuable commodity for human life: Food . . . who wouldn't want to do that?