OK, all just my opinion / theory:
The board and the planchette are just made of wood and cardboard, or whatever. I agree about that. This is why some people can make their own ouija boards. There’s nothing special about the ouija board itself.
The spirits (or ghosts, or the dead, or whatever) are unable or unwilling to just move the planchette around by itself. It wouldn’t make sense, and it probably wouldn’t be possible.
Humans all have a certain part of their brain associated with things which science can’t explain. I belive that to a certain extent we are all a little psychic. I won’t go into too much detail here, as it’s a whole other debate, but I cite things like deja-vu as evidence of this.
The spirits which want to communicate used to be people just like you and me, they just died is all. Whatever energy (for want of a better word) they use to get their message to us, find the path of least resistance through other people. We are the channel that they use to communicate with our world. Psychics who communicate with the dead do this without the use of a ouija board, adding more weight to the theory that the ouija boad is only a conveniant tool.
As channels for this energy, we need to be able to see the letters on the board. When the planchette moves, yes it is being physically moved by the people touching it, but it’s just possible that the signals from the brain that tell us where to move it are coming from a source from somewhere that we can’t explain.
In short, the ouija board is an item of convenience for the less psychically enabled, to convey simple messages from spirits. In order for it to work, I think you have to have a belief that it can work, and you need to be prepared to open your mind and let the spirits guide you.
This is all just theory, and there is no way to prove it. However, I have yet to see someone disprove it.