Because it's pretty well established that outside of the most abject poverty, relative deprivation is a more significant factor driving experienced disutility than absolute material condition, contrary to the naive assumptions many people make that increasing individual absolute material condition increases individual utility independent of external context.
There are, of course, people who don't believe it works that way, people who don't want it to work that way, and people who don't want other people to believe it works that way because such belief threatens their own advantaged position.
However, it does seem to me that that you still haven't addressed one of the main points you're responding to, that we can have perfect equality where everyone lives as the average person below poverty line today. Would that be a better society considering your arguments?
Well, no, we probably can't. Inequality can be mitigated, but it can’t be eliminated, especially durably.
> Would that be a better society considering your arguments?
That's really an objectively unanswerable question for many reasons. You can't really measure, on more than an ordinal scale, individual utility, or aggregate it, except by interposing non-objective judgements. Would it be better for lots of people currently above the poverty line? Maybe, depends how far below the poverty line the equal society was.
Which data are you talking about? Because on the one hand, yes. On the other hand most of the data I'm aware of is very context- (especially social-situation-) specific. I don't think there is evidence that the simple existence somewhere out there of very wealthy people is upsetting.