Because that's how it keeps going. I have tried time and time again to ignore the little danger signs from White people and kept getting burned by some, even by self-professed leftists. Not by all, of course, but by enough to be wary. It's unpleasant; this hypervigilance around White people is a real burden and I wish it hadn't been forcibly inflicted on me, but that's what living in a White Supremacist society does.

I find it helpful to differentiate between "White" and "white" people.
Capital-W "White" means the person exhibits a lot of the toxic settler-colonial, imperialist, and casually racist behaviors that Amerikkkan society perpetuates.

Lower-case w "white" means the person has skin tones that make them look like the Capital W's but they don't do that shit. They're actively antiracist, they're vigilant and purge any trace of white supremacy from themself, they're learning, and they support people of color in our fight against white supremacist fascism.

It's not about skin color, and it's not even essential to who they are -- White people who rehabilitate themselves can become white people, that is, people whose skin is light but aren't being toxic or shitty in any way.

A pretty basic litmus test I've found is checking how they feel about American Imperialism. If I say, "Joe Biden belongs in prison for what he did to Iraq" and they agree, then they probably understand on a visceral level that brown people are people. If they argue, that tells me they don't actually value our lives -- because it is fundamentally impossible to care about nonWhite lives and also be ok with any of these genocidal fucks.