Because there's not enough reward for high risk. VC (and PE & Sovereign Wealth funds) do make multi-billion dollar investments, but mainly in areas like energy

Because there's not enough reward for high risk. VC (and PE & Sovereign Wealth funds) do make multi-billion dollar investments, but mainly in areas like energy where they can get a huge pay-off. Say you wanted a fund to invest $20bn for 100% equity, generally early stage investment funds look for at least a 10x return, so they'd be looking for a company that can be worth $200bn. The biggest cell provider in the US in Verizon, and their market cap is around $100bn. To justify the investment you'd need to build a company that pretty much controls 100% of the US cell phone market. And even then there's no upside for the investor, they might get 10x if they're amazingly lucky, there's pretty much zero chance of 20x or 100x return.