Snakes eat other animals. Large dogs consume mass quantities of food. Exotics need a variety of specialty foods. Some critters will go absolutely cage crazy if they aren't provided with constant enrichment, exercise and or stimulation. Some need to be maintained at certain temperatures.
Some animals are solitary animals.
You may laugh, but even some commonly kept pets are illegal in some places. It's not only the wild and dangerous animals that are regulated. Animals such as hedgehogs, prairie dogs and ferrets are frequent targets of animal bans. That cute 12 inch boa constrictor will reach 10 feet if cared for properly. Many snakes get much bigger.