[AngryData]

Pigs breed like rabbits so their evolutionary path hasn't been to ensure individuals survive at the highest possible rate, their path was to have a dozen babies at a time so that even if 80% of them get killed or eaten, their population still grows and thrives. For a farmer losing 20% of their pigs because the mother sat on babies and suffocated them is a massive loss of money, for a wild pig it doesn't matter as much because 3x more will get eaten by predators and there is already another dozen on the way within a week or two of giving birth to the first litter.

Some of the loss likely is due to keeping them penned up, however there are also losses for not keeping them penned up and letting baby pigs run among a herd of many adult pigs, some of which will attempt to kill piglets, especially females who have not had piglets yet. Pigs can be absolute viscous as hell and will readily eat other living animals if they think they can get away with it, including other pigs, and some mother pigs have been known to cannibalize their young even under ideal situations. Pig farmers have themselves been killed by pigs from passing out or getting knocked out in pig pens and the pigs seeing them as a free meal not to be wasted.