It's incredible how many wrong answers can be given with such determination that they are correct.
Why do we write G-d?
It is written to refer to the G-d of Israel.
Although G-d is not a name but a designation, it is still referencing that G-d.
It comes to a matter of respect. If I can write it, it can be destroyed. There is no superstition involved, only respect. Even inadvertently, I choose not to risk desecrating that which refers to my G-d.
And to 'fireball' thanks, but we (Jews) do not need your permission to desecrate reference to our G-d. If you want to desecrate yours, fine. But don't speak for us.