This happens because:
1. At some level of company management, far removed from the animals, the animals are just inventory...a line on a spreadsheet whose importance is tied to profit. The people who "call the shots" at these large companies never come into contact with the animals so they are removed from the suffering of the animals. Many companies talk a good game when it comes to animal care (it's good PR anyway). And some companies even do a decent job of caring for the animals they are selling. But consider that even if a store properly cares for the animals it sells, it has little or no idea what condition the animals are kept in at the breeders. How many sows are kept in too small cages, back-bred until they can no longer support a litter just so the stores can have fresh "inventory". How many breeder pigs and babies do you think receive any kind of vet care? And this is just the suffering that takes place BEFORE the pigs get to the store. I don't know a single pet store that screens their buyers to make sure they can provide adequate care for the purchased "inventory". So even the best possible store supports the suffering of animals at breeders and with ill-prepared owners.
2. Hiring qualified, caring individuals in the store is not aligned with the goal of the store...to turn a profit. Yes, there are many many good, compassionate animal lovers who work at these stores and some even do a damned good job caring for the animals. If you are one of these, then I thank you. There are also many many careless, poorly trained, or downright cruel employees working at pet stores. The fact that good, caring, and competent people are sometimes hired, is in most cases just a happy accident. How many stores provide zero, inadequate, or downright incorrect training in the care of their animals? Compassion for animals is not one of the qualifications most pet stores are looking for when hiring their employees.
Sorry for the rant and for getting all melodramatic. And I know I'm preaching to the choir here at this forum. But I've seen all these videos before and they do really tug at your heart-strings. There's been too many threads and stories about how folks just had to "rescue" the pet that they saw in the pet store. Now, many people don't have this information and innocently make the mistake of purchasing from the store. But if you've read this thread then consider yourself informed. If you decide not to purchase your next pet from the store or if you can convince a friend not to do so, then we have truly rescued an animal somewhere.
</rant off>