Askreddit, but without degeneracy

It's because the Jewish religion is fundamentally tied to particular genetic lineages (Ashkenazi and Sephardic). You can practice all the other strictures of the religion, but without proof of matrilineal genetic descent from a Jewish tribe (meaning in practice: proof that your mother was also genetically a Jew, whether or not she practised the religion), the Jewish religious community will not recognise you as a "true" Jew.

This is a dim take.
 
The idea of a nation (especially outside of the united states, but especially as a biblical sense) relates to a cultural identity, and not just lines on a map.
 
The borders of a country exist(ed) to protect the cultural norms of the ethnic group inside them.
 
When the jews were evicted from Israel, they didn't have borders to protect their cultural (ie. religious) identity. The solution was to create an insular, stateless community. The reason judaism survived as a religion is because the Ashkenazi only married other Ashkenazi; the Sephardic only married other Sephardic. Because of this cultural norm, people of Ashkenazi/Sephardic lineage were raised with jewish cultural beliefs, traditions and norms. Even if they weren't/aren't practicing jews, people of this lineage are generally raised with a set of values that reflect jewish/internationalist/communal/stateless (as opposed to nationalistic) ideologies.
 
If jewish wasn't an race, it wouldn't be a religion.
 
Also: See Whoopi Goldberg; she recently got her pp smacked for saying the holobunga was white-on-white.