Saying “white lives matter”. How is that anti-black? By that logic saying “black lives matter” is anti-white.
If you’re seriously asking, it’s because these statements don’t exist in a vacuum with no context. “Black lives matter” is a social movement attempting to highlight systemic oppression against black people in the USA, especially but not exclusively from police. “White lives matter” is meant as an undermining response to that social movement, not a statement in a vacuum, and has neo-nazi origins.
We could definitely choose to take Kanye’s explanation about being upset with white people wearing BLM t-shirts at face value, but I suspect a lot of people won’t choose to do that, and that’s why they’re interpreting his comments as anti-black.