To own a big company (many are TOO big, incidentally), it is not required to be a billionaire. You've got cause and effect reversed.

Being a billionaire results from owning a highly profitable company, or companies, and involves too great a share of that profitability rising to the top. Socialists believe workers ought to own companies and share the wealth produced.

No one needs to be a billionaire. It is the grotesque and greedy accumulation of wealth (and under Citizens United, power) by one or more individuals. Sheer greed.