If a cat keeps meowing while in a cage, it might be hungry, thirsty, or simply not used to being in the cage. Cats are free-spirited animals, but if the owner soothes the cat and lets it out gradually, the cat will slowly adapt.
Cats are freedom-loving animals. Some cats meow incessantly once they're in their cages, which is related to their personality and not necessarily due to some other reason. Letting them out might stop the meowing. So, keeping cats in cages isn't necessarily a good thing; you also need to learn how to interact with them and help them relax.
