It's easy to guess, if you know both Japanese and English well enough; it's probably a translation error.
The original Japanese name is "星のドラゴンクエスト". What matters is the Japanese word "星".[note 1] It is a notion that encompasses stars, planets, comets, etc. More precisely, any celestial bodies you see in the night sky (except the moon) are "星". It is by far a more common word than 恒星 (a star), 惑星 (a planet), and so on.
This is the first problem — Japanese treat stars and planets as one in the daily language. But of course they do know stars are not planets. The second is, well, it's embarassing, poor quality dictionaries. "星" is often translated to "Star" and vice versa.[note 2] That is likely to be the root of the current problem.
I also mention one more subtle point. The Japanese language does not distinguish sigular and plural. For the first two years, there was only one planet in the game, and two more have been added later, as of 2020. So in Japanese, the original game title has not changed, and it's ok. But if it were English, which had been appropriate, of the "Planet", or "Planets"?
- [note 1] "星" is romanized as "hoshi". It rhymes with "got it", but the stress is on "it".
- [note 2] For example, if you look up the word "星" in ウィズダム和英辞典, a popular Japanese-English dictionary, the entry says "star", and the entry of "star" in ウィズダム英和辞典 reads "星; 恒星 in astronomy" (the latter word means a star). If you feed "星のドラゴンクエスト" into Google Translate, it'll return "Star Dragon Quest". Sigh.
-  See for example this Japanese dictionary site page.