This is not least because a USB device needs to be able to identify itself so the computer knows what is attached. To do this you need a signal converter somewhere in the system. This can be in the converter cable/connector or in the more recent PS/2 devices is actually built into the keyboard/mouse as manufacturers realised that USB was becoming the connection standard and shipped their equipment able to function as PS/2 or with a passive adapter (because the active signal conversion was happening inside the device) USB. This led to a bit of a myth that all PS/2 female to USB A Male adaptors would work for all PS/2 devices but passive PS/2 >USB converters will generally only work if there is signal conversion built into the device already. If you plug your PS/2 device into a passive USB adaptor and the device is not recognised then it doesn't have a signal converter built in and you'll need to get a connector that does. A secondary consideration depending on the age of your PS/2 devices is current draw - PS/2 devices were designed to pull the current they needed directly from the motherboard but USB devices are restricted in total current draw by the capacity of the USB hub.
