Why you can't just buy an arcade DDR platform and hook it to your Playstation...

It's a very simple matter of pad electronics. Each individual panel has 4 analog pressure sensors. They don't act as normal switches, but rather, send an analog signal that tells how hard they are being pressed. These sensors are all hooked to an analog I/O board inside the platform itself. This analog board can be programmed to respond with different sensitivities. This analog board is then connected to the cabinet itself, and it uses serial data transmission of some sort. It might be similar to the PSX protocol, since they seem to use the same number of wires, but in all likelihood, I'm sure it's different enough to be incompatible.

To review:
-The pad does not just have simple switches that you could wire to a normal PSX controller.
-The I/O controller that does read the sensors uses a custom serial transmission protocol.
-Unless you're an electronics engineer, it ain't gonna happen.