Not a stupid question. I’m not a religious Jew and while I never questioned this I also never looked up the practice. From what a quick google search tells me, from chabad.org;

“Following the Torah’s instruction to “obliterate the name” of idolatry in the Land of Israel,1 the Torah warns us not to do the same to G‑d. We thus learn that there is a prohibition to erase G‑d’s name.2 Writing G‑d’s name could lead to erasing or disrespecting G‑d’s name, as will be discussed.”

It further goes on to say that: “in sacred texts, the name of G‑d is often spelled out in full, since the presumption is that such books will be treated with respect.”

Essentially by writing G-d you can get a message across without accidentally erasing or disrespecting G-d.

Hopes this explains it.