There aren't enough wires on a USB connector to
allow you to speak the PS/2 protocol.
How many wires are necessary to speak the PS/2 protocol? How many pins
are there on a USB connector?
In front of me right now I have a Microsoft IntelliMouse Explorer USB
and PS/2 Compatible mouse, and a Cherry G84-4100PTAUS compact
keyboard, which both have USB plugs on their cables and both work with
passive USB to PS/2 adapter plugs.