All it is is whether a compound word is common enough.

It starts in speech when the words are repeated next to each other often enough they start being thought of as one word. But can’t be shortened.

If, in context, every time we said farmer we ended up saying dirt farmer. It would become compound. But in reality we’d just end up saying “farmer” when the context makes it clear. You’ll see this in writing about farming all the time, initially stating the type of farmer then just saying farmer.

Flag pole started out separately, but in some conversations it would become one object. Every time we talked about the flag pole it would be one word, flagpole. But saying just “pole” would be ambiguous. There are other poles around.

It trends towards shortness, if context allows us to drop a word altogether we will, if it doesn’t it gets compounded abbreviated.

No formal rule for this at all, but that’s the way it happens. People try to say things more efficiently without confusing meaning.