That has to do with history, Transport Tycoon Deluxe wasn't able to transport multiple cargoes per ship, and mail only via slow ships, so OpenTTD will not get it.
On the About page:
OpenTTD is an open source simulation game based upon the popular Microprose game "Transport Tycoon Deluxe", written by Chris Sawyer. It attempts to mimic the original game as closely as possible while extending it with new features.
Since ships with multiple cargoes were not in Transport Tycoon Deluxe, they will also not be in OpenTTD by default. However, a change in which NewGRFs could add such functionality would be fitting within that attempt, as OpenTTD would still mimic the original with the default vehicles while providing a way to extend them through NewGRFs. That would require someone who thinks that is a good idea and can code (or convince someone to code it), and get it included by the developers.
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