Because there's a whole continent of other delicious animals that you can treat as livestock that won't kill you in numerous horrible ways. Hippos are nasty, aggressive creatures.
As well as the logistics of suddenly having a huge carcass that needs to processed right away. Butchering a cow is hard work, now think of how much more effort would be needed to deal with the additional mass of skin, bones, organs, and the meat of a hippo. All before it spoils.
This doesn't even begin to address the issues of attempting to manage live hippos before getting to the slaughterhouse.
Hippos are hunted for sport though -- google says the meat tastes like beef but gamier.
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