Because instead of everyone sharing in the country's prosperity, all the growth for decades has gone to a small percent at the top.
Your question is absurd. It's like asking, if a company cut salaries by $100 million to give the CEO a $100 million pay raise, 'how is that at the expense of the workers?' You can't use common sense because you are an ideologue.
All you can do is rant about semantics in the other thread, or your dogma about the role of government, all you care about is your dogma, not the effect, no matter how oppressive inequality becomes.
It's sick. While it's not a 'zero sum game', it is worth understanding the ways it is for policy, and how yes, government CAN and SHOULD look at distribution of wealth as a top factor in tax rates.
When those that want to share in the prosperity start contributing to the cause of it, they'll get some of it. You want those that contribute nothing to the pot to draw from it while expecting those that have to offset the freeloaders paying more.
Your mindset is for those that do nothing to get a portion of what those that did something earned hoping that it will motivate the do nothings to do better. Hasn't worked and never will. Why do you oppose people actually doing something other than sticking their grubby paws out to get what they need?