a usb-serial adapter emulates a serial port... so the computer needs to recognize that the usbdevice IS a serial port... which is something you can't easily slot into the OS, since you'd need to install a "driver" to do it.
when you inset a usb device your computer asks what it is. "imma usb to serial port device" it says "sorry, i got nothing here for you kthxbye". whereas the touchscreen says "imma 3m microtouch touchscreen" and the computer goes, "yeup i got your files right here you can speak to me"
a usb to serial doesn't just change from one pinout to another, it's a whole different protocol.
can you adapter a parallel to 25pserial to 9pserial to ps2 to usb? yes.
does it work? no.