Because parasites can’t survive outside the host.
Funny you say that. I was just having that exact conversation yesterday with a friend.
My friend pointed out that for black people, all of the items and requirements for life are provided by white people: Goods and services, emergency response, etc: all are either created/built/concieved/invented by white people, or are entirely dependent upon the taxes of white people to continue. Without the "Invisible White Hand of the Taxpayer", black people would quickly return to the lifestyle of their ancestors and revert to filth and squalor.
On the other hand, black people provide absolutely nothing that white people need. Every single thing they do can be easily replaced.