Because by scripture you cannot become a Jew. You are born into it.
You can convert. Read the book of Ruth.
Not according to the Rabbi's. Many call me a Noachide. I am not a Jew, but my actions prompt this statement a lot. There are a few Christians I know of that converted to Judaism. They live in the MeaSharem area. They are considered not noachides, but Jews in the full sense of the word. It took a few years with their Rabbi's to earn the title. They are ultra orthodox now.