By Trevor Hunnicutt and Jarrett Renshaw WASHINGTON (Reuters) -U.S. President Joe Biden, under pressure over sky-high gasoline prices, on Wednesday demanded oil refining companies explain why they are not putting more fuel on the market as they reap windfall profits. Biden wrote to executives from Marathon Petroleum Corp, Valero Energy Corp and Exxon Mobil Corp, and complained they had cut back on oil refining to pad profits, according to a copy of the letter https://docsend.com/view/qpg3e8a2s3fbxi3a seen by Reuters. The letter was also sent to Phillips 66, Chevron Corp, BP and Shell, a White H…