Because the old fisherman went out to sea to fish, but there was no goldfish. Goldfish is Poseidon's daughter, and promised to satisfy all the wishes of the old man as long as she was released. The kind old fisherman let her go without any reward. When the old fisherman came home and told his old woman about it, the greedy old woman began to force the old fisherman to make demands on the goldfish again and again, such as wooden pots, wooden houses, money and status, and eventually she would become a maid to wait on her. In the end, the goldfish couldn't stand her greed.