This is due to the word "star" acting as an adjective rather than a noun. You may have seen "Five stars shining in the sky" when a noun, like hotel, is added after the term "star".
You would say, "I had dinner in a three-star hotel."
You omit the phrase "three stars hotel."
You also say, "I want five rupees." Nevertheless, if you add the noun "coin" or "note" after the word "rupees," you would say, "I need a five-rupee coin/note." You do not say, "Five rupees note"
Joggers refer to a "twenty-mile marathon" rather than a "twenty-miles marathon."