Because the wildfish had to face many more adverse conditions to get big enough to head out to sea. The weak are removed at all levels by disease, predation, enviornmantal factors whatever. Hatchery fish are hand raised until they are big enough and you do not know which ones would have died in natural conditons so they get a hand up in returning to spawn. So ultimately if they are allowed to breed with wild stock they water down the gentics. Ultimately leading to more mortality and the system crashing if an event happens that pure wild fish would have been self selected to withstand.

Your twins example is not valid as an example. You would have to set one twin free to fend for itself in Bangledesh to be comparable?