Because while chicken wire does an adequate job of keeping chickens in, it is virtually worthless at keeping predators out! Raccoons can reach right through the wire and grab a chicken, if they haven’t bothered to just totally rip the wire apart. Rats and weasels can go right through it. So can small wild birds...you’d be amazed at how many mites and lice can hit your flock via an over abundance of sparrows getting in, not to mention how much their feasting will add to your feed bill. 
Edited: Hope it’s okay that I answered....I wuz jest setting doing nuttin’!

Edited: Hope it’s okay that I answered....I wuz jest setting doing nuttin’!

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