I wondered, if おはようございます is okay then why not おはようです?
Grammatically, おはよう is a contraction of おはようございます, which is a Western dialect form otherwise should be おはやくございます in Standard Japanese. Since おはよう as such is not a valid word except for this fixed idiom, you can attach nothing to おはよう grammatically.
Is it a different level of politeness? Is it a regional thing or is it just weird?
Contrary to what was said in the previous paragraph, strictly speaking, です is not prohibited to be attached to any phrases to make polite direct quotation out of them, and you can certainly treat it as a simplified polite ending by taking over this grammar. This usage is neither conventional nor standard in Standard Japanese, so you can think it of a mildly broken grammar. A girl speaks in off-point grammar is, some kind of, sign of cuteness.
Additionally, some dialects indeed use this versatile です as their normal way of speaking. See: