Because large organisations have demonstrated over time that they are fundamentally incapable of managing their certs effectively.

They usually grow enormous bureaucratic theatre around long-lived cert renewal, leaving them unable to do rapid rotation when it is required. And rather than getting their shit together, they prefer suing their CA to avoid revocation.

Given that those organisations 1) are crucial for day-to-day life (like banks, or the government), and 2) would have a giant blast radius when their certs are compromised, and 3) won't voluntarily change, the only remaining option to keep the ecosystem healthy is to force them by making complex rotation processes extremely painful and cumbersome to keep.

Hence: automated renewal, and boil the frog by slowly lowering the validity period.