If Jews don’t proselytize and believe that even righteous non-Jews can achieve heaven, then why is there so much of an emphasis placed on maintaining a "pure" Jewish lineage?

Started reading up on Judaism just for fun. I read Maimonides in college, but other than that, I’m pretty weak on Jewish theology so I was wondering if you can answer some questions for me.

I’m a former Muslim, now atheist, so I guess I approach Judaism from that perspective.

Because of that, some things just don’t make sense to me.

For example, proselytizing is HUGE in Islam. There’s even a term for it: “dawah.” I was surprised to learn that proselytizing isn’t really a thing in Judaism.

Whereas in Islam the focus is on bringing others into the fold of Islam, Judaism’s attitude seems to be “you’re not one of us, and you don’t really need to be one of us”

But at the same time, there’s a huge emphasis on having to marry within the Jewish community if you are a Jew. I guess what doesn’t make sense to me is that if non-Jews can achieve salvation by being a good person, then why is there such exclusivity in the Jewish community? Why isn’t intermarriage freely allowed? Why do they seem to shun non-Jews?

In Islam, if you find a non-Muslim to marry, you teach them about Islam and then they are welcomed into the community if they truly believe.

I know that conversion to Judaism is a thing, but it doesn’t seem to be as widely accepted or called for as is the case in Islam. And why would it be? if salvation can be achieved by being a non-jew anyway?

I believe Maimonides’ opinion was that you had to convert to Judaism to achieve salvation (correct me if I’m wrong), so I’m talking about the modern view here.

Can someone who knows more about Judaism clear my confusion?