I've been thinking about this a lot this past decade or so, and it is an interesting question. One thought I've had is that a perception (real or not) of inequality is baked into our species as being bad, particularly when it gets extreme. Why might that be? Well, if my neighbor generally has more resources than me, it's usually not that big of a deal. You maybe have more or better food, or some nicer things, but if my needs are met and I have things I can enjoy, I don't get worried.

Now, if you suddenly have a lot more than me, say you have enough to provide for other people in a meaningful way, I start to get concerned. Why? Because you don't just have more than me, you start having power over me. And if I don't stop inequality increasing, that could quickly get to the point that I can't do anything about it. Your power can become absolute, for all practical purposes.

I think humanity has learned culturally, if not genetically, that really bad situations arise if someone has way more than you. Power corrupts and all that, and gross inequality becomes power.

Of course, these days, it might not be "real", meaningful inequality (Jeff Bezos has a lot of wealth, but it is wealth tied up in Amazon, mostly, and the US Government has far, far more wealth), but the perception is there, and the perception is enough to trigger concern.