What Jan and Arne say makes sense. but still leaves me a little puzzled.
A 48 volt system is still going to need more than 48 volts to charge the batteries. Here is an example (Cinderella's Solar Panel Installation (sailingcinderella.com)
This boat has two SolarWorld 345w panels in series and installed on a bimini over the cockpit. Evidently this provides nominal 80V. So, even limiting the battery bank and motor to 48 volts, there are still significantly higher voltages there.
I suppose, as Jan says, it is partly a matter of balancing safety and economics.
Its just that I haven't come across any mention of this by manufacturers, or people who have installed their own motors (apart from the occasional comment about the need for care with high voltages). I wonder if there are any other reasons which relate to small boat electric engine systems, which makes 48 volts seem always to be the preferred system.
Edit: I just did a bit more googling and found that the human skin, when wet, can have resistance as low as 500 ohms. That means 50 volts would be enough to push 0.1 amps through you which, in the right circumstances, is enough to kill. So, I guess that's enough reason to be wary of higher voltage battery banks.
I'm glad I asked. It still puzzles me, if that is the main reason, why it is not spelled out more clearly in the literature.