They think their religious beliefs come from (lehavdil!) "god" just as yours come from HaShem. They are not intentionally following something they just pulled out of their jammies any more than you are. As long as they sincerely believe this, it is cruel for you to demand them to not obey their religion and try to convert everyone. You don't want them doing that? Then convert them to Noachism.
Judaism is very much against proselytizing others to become Jews. It is very difficult to convince an Orthodox rabbi to teach you the precepts of Judaism culminating in conversion. I am not even discussing the "Become a Jew in 10 Days, Just Add Water" nonsense of the non-Orthodox. Their "Insta-Jew" stuff is not in accordance with Jewish law and the conversion would not be recognized in Israel.
You're being a bit disingenuous here. Judaism does not proselytize non-Jews to convert to Judaism, but it very much does want non-Jews to convert to the Noachide Laws. I notice that you never mention these. You are raising two false alternatives: sincere belief in the exclusive truth of chrstianity (G-d forbid!), or else accepting all religions as equally valid. The Jewish mission to all non-Jews is to convert them to the Noachide Laws. Non-Jews won't make it into Ha`Olam HaBa' unless they do this.
You have never known what it is like to change religions once you learn the Truth. Someone like you and someone like me (a "convert" of sorts myself) will only ever talk past one another.
I'm done here.
A question of genuine curiosity: What is Ha Olem HaBa’?