why do they call the rooks a pig?

why?

The first time I heard of "blind pigs on the seventh rank" was back in the 70s.

Wikipedia: "The two rooks are sometimes colloquially referred to as 'pigs on the seventh', because they often threaten to "eat" the opponent's pieces or pawns."

must be a non english thing

Because they eat all your pawns.

News to me.

If you have both opponent's rooks in your 7th rank (or 2nd if you're white) it's a bad situation... They (the rooks) tend to eat like pigs. Very nasty...

They are bulky-looking, usually penned in for a long time, navigate obstacles rather clumsily, and when they get to the 7th rank all the unmoved enemy pawns resemble a horizontal trough full of feed.

They don't move, except to eat!!

"paging Huey"

http://www.chesskid.com/article/view/blind-swine-mate

Also, "The two rooks are sometimes colloquially referred to as “pigs on the seventh”, because they often threaten to “eat” the opponent’s pieces or pawns." - http://aboutchess.mustofpain.com/rook-chess/

or, for black, 2nd

pigs on the seventh 

bit*hes on the eighth

Because they're immune to snake bites.

who are ''they"?

Let me hijack your thread to ask a more serious question: What is the name for a rook doing the same thing on the 6th rank?

Nice contribution.

Pigs because they have so much fun eating pawns

Not a single credible historical citation, and yet Edward Winter addresses this question (and the paucity of sources) in Chess Notes 3494, 3525, 5160, 6108, and 7003. See https://www.chesshistory.com/winter/index.html