Because there are things you should not see
A typical slaughterhouse kills up to 1,100 pigs every hour. The sheer number of animals killed makes it impossible for them to be given humane, painless deaths. Because of improper stunning, many pigs are alive when they reach the scalding tank, which is intended to soften their skin and remove their hair.
The U.S. Department of Agriculture (USDA) documented 14 humane-slaughter violations at one processing plant, where inspectors found hogs who „were walking and squealing after being stunned [with a stun gun] as many as four times.“
According to one slaughterhouse worker, „There’s no way these animals can bleed out in the few minutes it takes to get up the ramp. By the time they hit the scalding tank, they’re still fully conscious and squealing. Happens all the time.“