Yes, they technically did. But they are congratulating themselves on an 18% gain, having invested in early 2018. The S&P 500, with reinvested dividends, is up 22% over the same period.

Upside for Uber is higher than S&P at this point. Could easily go above IPO price given the general upward trend + their good earnings.

Then they could have bought S&P500 first, then buy into Uber now

That’s just called hindsight. If anyone knew at the time of decision what the perfect investment was we’d all be billionaires.

Right, we don’t. But no one buys the S&P500 because it’s the perfect investment. It’s diversified. Much less risky then buying an individual stock like Uber.

You're wondering aloud why a VC firm doesn't just buy the S&P. I think you can figure that one out yourself.