It’s not so much that Jewish people are forbidden to write out God’s name (in fact, there is no Jewish law stating they can’t), but that the more God’s name is written out, the greater the chance for it to be defamed, erased, or thrown away. The key is that God’s name is supposed to be revered. Many Jewish people see it as a sign of disrespect to write God’s name, so they will abbreviate it “G-d.”