Jewish friends? help me to understand?

jewish rabbis say there is not obligation to convert to judaism but as non-jewish we are obligated to do the Noah Laws, I understand judaism doesn't obligate convert anybody for salvation but what is the purpose of having gentiles or people who are non in judaism to do the Noah Laws? like idolatry and stuff like that, if those laws are for the humanity why do jewish have are expected to preach that? for example a christian is idolatry because the seven laws of Noah forbid any image of God or to worship any God beside him, can you explain what is the purpose? why can't they be full jewish?