I would add another reason - connectivity. Seaplanes ruled the roost at a time when there were very few airports. Then some airports developed around sea plane ports (eg LGA).
Now, must be 99%+ of commercial aviation uses "land" airports. So how do you connect from one system to the other? There are a few examples (SEA, iconically enough) where this can be done, but otherwise you need a land transfer on top. Just not worth the hassle most of the time.
You could also ask "where are all the car ferries in the Caribbean, when there are so many cruise ships". And you'd also have answered your own question!