I think the reason they have so many is clearly this is where Merlin make their money, apart from the initial build cost and general day to day costs you don't have any major costs throughout the life of the attraction, compared to a theme park. You also dont need to add new rides every year, and like most sealifes, they just move fish and stuff between each one.
And by adding a group of attractions together, they can do what they do in Blackpool, price them on their own, but do much better deal rates, which I imagine 99% of people buy.