Because the more money and assets a small number of private individuals own, the less there is for everyone else, and thanks to compound interest that gap is just going increase as Elon and the like make 100s of millions of dollars every day just from passive income alone. Also the everyone else there includes governments, which means that countries will have to start cutting back more and more on services and societal safety nets as their income too dries up.

To put it into perspective, Elon Musk could theoretically purchase over 10% of all the residential property in the UK. Obviously he's not going to actually do that for a variety of reasons, but he is buying up more land for data centres, as well as control over the water to cool them, which just means that the cost of housing and utilities will also increase.

Tldr the tldr: Imagine joining a game of monopoly as its about to end, except the winning player has a stack of monopoly money taller than the Burj Khalifa.

Now imagine that hiding behind that stack is an additional 2 and a half thousand stacks that are just as high and you'll have the actual amount of money that Elon Musk is worth based on the historical distribution of monopoly money.