Why do people get so angsty over the idea of racial purity?

The absurdly stereotypical retort is something like BUT MUH SCIENCE!!11!!!!

Science aside, why do people care if a country like NatSoc Germany want's to implement policy based around a notion of racial purity?

It seems like it mainly a preference in either direction - that is, diversity and racial diversity are as equally unscientific as racial purity but rhetoric regarding diversity is seen as the moral high ground. Why? There's often more diversity among in-groups than between out-groups, so diversity is just as pointless - strictly scientifically speaking - as racial purity.

It's like, ok I get it, you were conditioned from birth to hate anything and everything having to do with NatSoc Germany and they were big on the racial purity thing, but all the propaganda aside why do you - or anyone else - care? Further more, why is it such a REEEEEEEE inducing topic? It isn't just denied, it's viciously attacked like the plague.

Of course I'm sure most of us know (((who))) is behind the muh diversity bullshit but does anyone actually stop and analyze why they think racial purity is a horrible idea?