Expected Market Return

I can’t figure out how the expected market return is calculated. I understand it’s function in CAPM and in determining beta. But I don’t see how any value for expected market return, market covariance with your stock, or variance of the market, that you’re given is reliable. Has someone ever calculated the expected return for each asset available, the SD for each asset, or correlation between every pair of assets? To me this seems like the #1 flaw of CAPM because subjective probabilities are needed to even generate variance!!!