Why don't animals care about their fitness?Talking about wild animals, their diets are still composed of whole natural foods. They still hunt for their food. A snow leapord must always be fit to get its natural diet everyday.
Coming to domesticated animals, their diets or exercise routine depend on owners choice. There have been incidences of overfeeding to pets. Some pets are even given sugary treats leading to obesity. But there can be other reasons like surgical sterilizations.
Obesity in pets
Feline Obesity: An Epidemic of Fat Cats