Part of the issue is simply availability. To find a variety of organ meats you usually have to go to a butcher or more varied meat market.
There are other issues to, and this is somewhat a matter of varying opinions from person-to-person. I know not everyone is quite as nitpicky about food sourcing as I am. I personally do not feel comfortable buying factory farmed meat (for my boyfriend or my cats), both for personal ethical reasons and for nutrition reasons. Factory farmed animals are usually fed diets high in corn and given regular antibiotics in their feed.
The high-corn diet causes the meat to be extremely high in omega 6s, and low in omega 3s and other key nutrients. Since cats only eat meat and therefore get those nutrients secondhand, I would assume it's especially important the meat they're eating is getting all the nutrients it needs from the proper plant sources. Cows and chickens aren't really meant to eat all-corn diets. The prey a cat would eat in the wild would consume a variety of grasses and other plants, and even bugs, resulting in better nutrient variety in the meat.
The overuse of antibiotics in our food have been linked to health issues in humans (allergies, bad reactions, antibiotic resistance). I don't know if the same can happen with cats, but I don't like taking the chance. I'm not sure how widespread the practice of injecting animals with hormones to stimulate growth still is, but that's another major historical issue that I've had with factory farming.
Factory farmed meat is also not processed in very clean or well-regulated conditions. I know cats can handle bacteria much better than we can, but having seen what goes on in a factory farm...I feel there's nothing "natural" about feeding that meat to my cats.
That said, not all grocery store meat is factory farmed or processed in such an inhumane and poorly regulated manner. You can find free-range, grass-fed meat in most major grocery stores now, and obviously there are a lot of stores out there now that focus on delivering these types of products (Trader Joe's, for example, gets a lot of their produce and meat from local farmers instead of from major corporations).
In summary, I don't think there's inherently any problem with buying meat in a grocery store vs. from a butcher or farmer's market, but it's easier to have a clear idea of where your meat is coming from and how it was treated and processed when buying from small local distributors, if that is an issue that is important to you.
Ultimately the raw meat you get in a grocery store is probably still leaps and bounds better than most commercial cat foods, so if it's your best option, I wouldn't stress about it too much. I am admittedly a little overzealous about food sourcing for myself and my cats after spending way too much time learning about the less favorable sides of the food industry.