Because the system under which wealth concentrate privately, called the free market, results in a steady increase of wealth, improving health, steady reduction in hunger and privation generally.
The system under which wealth concentrates socially, ie is controlled and redistributed by the government, tends to produce mass starvation. It is also associated with government controlled labor camps where political purge victims toil away until their own deaths, or are outright executed en masse.
These outcomes are, ultimately, a reflection of the fact that the former system is based on consent and the latter is based on forcing people into economic relationships they wouldn’t otherwise choose.
Under our current free market system, billionaires get their billions one consensual purchase at a time. Under socialism, the government gets is billions by wielding power at the end of a gun.