Rent seeking and corruption are two definitions that spring to mind.
The problem is the term greed is often used in a pejorative way as a proxy for success. If you work hard and achieve a lot and that comes with financial rewards, you are greedy by definition. Add in some snark about how the work hard part doesn't count somehow, and that's the modern critique of capitalism.
Oh, and rent seeking and corruption are because capitalism too and not at all human flaws seen to an even greater degree in every other economic system ever devised.
A billionaire could conceivably work much harder than the median person. However, they don't work 8000 times harder ($1 billion divided by the median net worth in America of $122k).
That being said, I think it's also unlikely that they brought a skill to the table that's 8000 times more valuable. My understanding is that people who become billionaires do so by leveraging capital.
part from my first job working for the government here in the UK all my other jobs have been for companies founded and run by private individuals. They've helped me put food on my table and clothes on my children.
On the other hand I do agree that when it comes to inheritance, rent seeking, financial manipulation, etc there's a lot to do to close loopholes and create a more equitable system. There is a good argument to be made that the main reasons inequality has risen are not good ones. I do not see those as fundamentally flaws in capitalism, they're certainly flaws but you get abuses in any economic system. They're not fundamental. The Netherlands is definitely a capitalist society, but they also have a wealth tax for example. There are good arguments for land value taxes, which some countries use to good effect. There are plenty of tools available to us.
Opposition to addressing inequality in the US tends to take the form of arguing that the rich worked harder, earned their wealth, are more valuable, etc.; therefore, it is immoral to redistribute that wealth. The argument that I am attempting to make is that the differences in wealth are so dramatic that it isn't realistic that they earned it in the sense that free market advocates are implying.
As a third world native I find corruption works better than everyone mindlessly following law under one self-righteous government becomes illegal under the next one, thereby making citizens life hell.