Because it makes us acknowledge and address our privilege. It makes us acknowledge the fact 'white superiority' eg white history being taught, white majorities in the media, white areas being more middle class, whitewashing everything is not the norm. You don't think about it, you don't question it, you don't understand it. Racism and inequality just isn't our fight and doesn't cross our mind.
White people hate talking about race because it challenges our privileged equality. We are fine with the way things are because according to our lives all races are equal - they are equal; all minorities are on parr and all white people are a step ahead and life is far more comfortable that way.
It makes us uncomfortable and it makes us acknowledge issues we don't want to talk about. When the statue of Edward Colston was toppled there was a lot of outrage at a 'historical figure' being tarnished. The same with Winston Churchill etc. These are figures who have been heroicised our entire lives and to challenge our norms is difficult.
Our country was built on the back of slavery, racism, invasion, acquisition of lands that weren't ours, the 'British Empire' was history's large colony of genocide and enslavement. We were never taught about that at school - we were taught about Rule Britannia, the great Empire that covered 1/3 of the world.
It's like blinkers you take off and when you have them off and see the truth laid bare it truly is sickening and in truth it makes you wish you were still wearing them, simply because of the fact it's easier. But once you take them off, you can't look away. You see every inch of it and it's unbelievable you didn't see it before.