Some of the other replies have given the basic reasons; the original Let's Encrypt team was influenced by this paper, which stated some of those arguments more

Some of the other replies have given the basic reasons; the original Let's Encrypt team was influenced by this paper, which stated some of those arguments more formally or academically: https://www.ieee-security.org/TC/W2SP/2012/papers/w2sp12-final9.pdf https://www.ieee-security.org/TC/W2SP/2012/papers/w2sp12-fin...