The risk of getting that wrong and accidentally destroying billions of dollars of shareholder value (don’t like the term, but it seems appropriate here) is very high (low likelihood, extremely high severity).
As a board member, why would you take that risk, when not taking the risk is so cheap? Ultimately I don’t think the risk weighted expected return on finding a cheaper CEO is greater than just paying Tim whatever number he pulls out of his arse. I suspect expected return is actually substantially lower (probably very negative) than what Tim gets paid.