Part of the allure of breeding fish is carefully choosing suitable parents, tweaking tank parameters to approximate natural conditions, checking in on the fish every chance one gets, watching the parents mate (if one is lucky), sitting and watching the fry for long periods of time, witnessing the wonder of birth (hatching) and worrying about the babies; how they are doing, if they are eating enough, etc.
Then one gets the satisfaction of feeding the fry several times per day and raising the fish. Watching them grow day by day, admiring their colors, intrigued by how they are miniature copies of their parents.
Part of the reason this is such a great experience is that the fish are ALIVE! One may see them swimming around and interacting naturally. One may see the fry grow into juveniles and then adults.
These are hard things to duplicate in a game and I have yet to find a game that is more satisfying than the actual raising of live fish...But then again, I have not played Fish Tycoon...