Group bookings may very well involve additional complexity that a specialist booking agent can assist with. I can totally envision, for example, that booking 10 "day rooms" in a hotel may take more effort than a few clicks on a web site. I don't claim to know anything about group trips.
The wire transfer thing can be problematic for a group trip, because the sums can cause eyebrows to go up, depending on the destinations and amounts.
One thing to keep in mind about many popular boats and companies overseas - an awful lot of their US business is group trips. Some popular boats especially get booked up for years mostly by dive shops and other groups. (Perhaps there is a bit of a chicken and egg thing in this, where people go with groups because it's easier for them, and the companies make things easy for groups because that's where the business is coming from.)
In addition, the group trips involve an economy of scale...which translates to more booking-certainty for the destination, and thus the likelihood of lower prices for the consumer.