I'm a white male, I know for me, and this is going to sound ridiculous, but it was actually really difficult for me to accept the idea that there were spaces and groups that I wasn't meant to be a part of. I think on some level I still haven't really accepted it. One experience in particular sticks out to me... I was dating a woman for a while, we went to a party together, and were doing out own thing for a bit. At some point I went looking for her and found her in a back bedroom with a bunch of folks. I just sort of walked in and sat next to her and joined the conversation.
Later on she pointed out to me that it was a room full of ladies, and that I changed the composition of the space just by joining. She pointed out that it's not actually that often that a group of ladies comes together like that at a party and holds space like that. I was totally shocked... the notion that I shouldn't just join my girlfriend wherever she was totally foreign to me.
I've struggled since then with feeling like I want to participate in spaces with folks who are different backgrounds/genders/etc than me (i.e. this right participation right here). I don't know how much of this is personal to me and how much of it applies more broadly to the folks on Reddit who you are talking about, but I grew up being told I could attain anything, and grew up with people who were competing to go to top schools and all that.... People use the word entitlement because that's exactly what it is. I felt entitled to everything. I felt entitled to have a great job and be respected and successful, but then I also felt entitled to be able to relate to people without those same opportunities, through activism and political awareness and all that.
That's kind of my guess for Reddit, that when you tell these people that there's some value to them being absent from a space, it just doesn't compute. It doesn't fit into the narrative of "I am a good person and I should be welcome everywhere and I want to be able to experience everything."
There's also a piece of it I've seen from white friends I have who are from more poor backgrounds, and also from immigrant backgrounds. Which is that when you are yourself fighting against some kind of oppression like that, you kind of have to separate yourself from the rest of your peers. You work hard to rise above it, and part of that often involves deciding that there is something about your character that makes you special and different from the people who were stuck in their position. But that means when you see oppression you're more likely to think "Well, everyone has some shit to deal with. They just need to rise out of that." My Dad, whose parents were documented immigrants, sort of has that attitude.
It also makes it really hard for them to stomach the idea that "all white people" do anything, or can be depicted in any way. They've spent so much of their life both feeling oppressed by one class of white people and separating themselves from another that they really can't see white people as a unified group. Any attempt to make a general characterization about races just can't even get off the ground with them.