The extremist Neoliberal version of capitalism creates massive income disparity in the society, with a very high concentration of wealth in the hands of a very few people, and a very large segment of the population in absolute poverty.
Aside from being unfair, it is simply bad economics. It creates endemic financial crisis.
A strong economy depends upon circulation of wealth in the market. People buying and selling. Disparity stops that circulation.
If 1000 people have $5 each, they will spend it in probably a thousand different places. But if one man has $5000 he is likely to make fewer purchases, probably on more expensive items, or he will buy shares or invest or put the money in the bank.
The 1000 people with $5 each contribute to the economy and keep the circulation in the market moving. The individual man with $5000 contributes much less and decreases the circulation of wealth in the market.