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.
Dogs grow up quickly too. As do lion and cougar cubs. Some pets need cages. No matter what, it takes some time to care for a pet. It varies from animal to animal.