The reason is that a cruise is a complicated product to sell. The main reason has to do with pricing. Inside cabins cost less than outside cabins. Cabins on deck 3 cost less than cabins on deck 10, which cost less than cabins on deck 12. Some outside cabins have an obstructed view, others don't.
Some cabins are bigger, some have verandas.
At certain times of the year cruises have lower prices. Some itineraries are more expensive. So, it isn't unusual for a cruise to offer as many as 36 prices for the same type of cabin on the same ship. You need a professional to guide you through this process and it takes some time. Cruise lines pay travel agents an average of about 14 percent commission to sell cruises for them.