In most cases those selling cruises for the cruise lines only have an interest in getting you to book on the line they work for.
In many cases they don't even know about much about their own ships, so then you have to contact other departments.
Over the years I've found the biggest source of misinformation provided about ships come from the cruise line office people themselves.
If the cruise line's sales people were as well trained as experienced cruise travel agents, I might recommend them more.
Free expertise of a professional to take care of my travel plans seems like a service I can't lose on.
People can get free or discounted services at such places as barber schools, and even dental schools. Some who make use of them walk away happy, others not so much, and choose to use a professional.
Booking a cruise, and assuring everything is taken care of (even the stuff you may not think of) is more complicated, and more expensive than walking into a shoe store, seeing a pair you like, and buying them.
What the internet does is allow you to find the services of really good professionals, no matter their locale.