Tbh the reason why people on the left insult ignorant white people is the same reason why everyone makes fun of and insults people who believe the earth is flat and etc... Because it's not an issue that can be argued, the earth is not flat and whatever you say it's not going to change that.

Same with white privilege, no matter how much you deny it that's not going to make it go away. It just exists and that's that, it's existed for hundreds and even thousands of years in many forms, as they are so many different privileges that you can't name them all. Big ones are race, sex, being hetero/homo, wealth, and many others that can change depending on where you live. The term socio-economic status is used to describe the majority of this as you can't name them all. Things like parents, peer groups, opportunity gap, different communities, good school systems and hundreds of others apply to everyone in different ways that benefit people. So obviously if you deny something as silly as white privilege people are going to make fun of you and/or insult you just like flat earthers because it's just hilarious how ignorant they are. White privilege is just the surface of privilege. That can be very counter productive but your not really viewing the situation right.

I was born in an upper middle class family. Born in a good household with supportive parents, a good community, I went to summer camps every summer when I was a kid, participated in outside sports leagues, with great peers, great school system, I'm white, male, Christian born (not anymore), straight, college paid for by my parents. I'm extremely privileged compared to most people and if you deny something like that your going to get made fun of. Even something as basic as going to summer camps is a great privilege to have.

If you read carefully you might be thinking that being white doesn't make you privileged as there are many others ones too. Even a black man is privileged as he is a man after all. It's not about excluding others, it's just about making fun of people that deny the basics of society.