But in this case the wires (or so I presume) go directly from the disconnect
to the breaker box. There is no chance for anything unfortunate to happen;

0 - Because it gains you absolutely nothing. The separate ground wire
is specifically for this purpose. So use it, and not the neutral, for
grounding purposes.
1 - Because "I presume" isn't good enough. What if they do not go right
to the breaker box, but instead have some other devices on them that
you didn't notice.
2 - Because your idea of "no chance for anything unfortunate to happen"
is mistaken. One of those wires could break, or a connection could
become corroded or loose, etc., all of which would result in live
current in your grounding system (i.e., unfortunate stuff hapenning)
3 - Because if done correctly and to code, it would later be safe to
add a device inbetween the disconnect and the panel without worrying
that somebody has done something stupid downcircuit (like connecting
the neutral and ground wires).

Connecting the netural and ground at the disconnect (or basically
anywhere other than the panel) gains you nothing at all, is risky, and
could be made even worse unintentially by someone who doesn't realize
the circuit was done wrong.
Tying the neutral to the ground gives you no advantage at all in the
case that there are no failures (broken wires, broken connections,
etc.). And in the case of failures, it can make things worse than they
otherwise would be.