Because those ephemeral LE certificates are such a great idea...
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Then, when you have only two or three big SSL providers, it's way easier to shut someone off by denying them a certificate, and see their site vanish in mere weeks.
However, short expirations severely limit the damage an attacker can do if they steal your private key.
And they avoid the situations where an organization simply forgets to renew a cert, because automating something so infrequent is genuinely difficult from an organizational standpoint. Employees leave, calendar reminders go missing, and yeah.
Short-lived certificates fixes these issues from an end-user standpoint.