Some cats can't stay indoors all the time without getting depressed. They just need to roam and explore.
You're really not supposed to let a cat just wander around unsupervised, it's really really bad and dangerous for them, but most people don't really educate themselves about pet care before purchasing a pet.
I don't think most people just "don't care" so much as they don't know, but there certainly are some scumbag pet owners out there. I have some pretty awful stories about cats being left out all winter and coughing up blood on cars, or ending up washed up in a storm drain, from back when I lived in an apartment building. LOT of scumbag pet owners there.