...this is called under capitalizing reserves. It's what took down AIG (and dozens of other less well known companies). It's basically using reserve float to make speculative trades. That means a slight adverse market move can leave the account under capitalized and unable to meet its claim obligations. If this was a sound investment play, every insurance company would do it. I reality, bonds rule insurance portfolios. Buffet has made his reputation on being risky by standards of insurance float investment.
It's taking on an unknown absolute risk in hopes of good return on expected probable risk. Buffet doesn't have the money to do this, neither does anyone else.