Capitalism might work reasonably well if everyone had the same income, since in such a world output would be distributed based on willingness to pay -- and those wanting and needing the most would have a priority.  As it is though the wealthy get most of everything and the poor often do completely without.

Therefore capitalist societies need income leveling mechanisms -- charities, graduated taxation, income redistribution schemes, and so on.  The problem is in democratic societies the vote is too often bought in various ways so that the wealthy over time dominate more and more.

In the "old days" (nineteenth century) Marxist thinkers thought this process must inevitably lead to a collapse and revolution.  I think what has saved capitalism has been that it can suffer mini-collapses with a partial reorganization, mitigated by technological progress, to stave off any day of reckoning.