Why do people let their dogs get so overweight?

It's sad. I had an uncle and aunt that had a dachshund. They fed him whatever, he was so big that his stomach touched the ground. Today I was looking on petfinder and came across a beagle in a shelter/rescue and it was 83lbs!(It was 93lbs when they got it, and lost 10lbs in a month). But I just don't know how people could let their pet get so over weight, and also not when they let them get that way and then do nothing about it/don't think there is anything wrong with it. I've had multiple people say my dog is "too skinny" when she's actually a healthy weight and fit. Not sure why people think dogs who are fit/in good condition are too skinny and accuse their owners of starving the dogs(this hasn't happened to me, but I know a few people who have been accused of it when their dogs aren't starved, at all).