If the cert lifetime is 5 years, Bill installs "just the first one" manually, solemnly swearing they will automate next time, and in five years nobody even knows who Bill was, Bill left three years ago. How do we replace the certificate?
If the cert lifetime is 2 years. Bill is still there or maybe somebody on the team still knows Bill's email address, but most likely the decision is taken to "just do it manually once because it's urgent" and we just recurse. This would have been the "obvious" choice when ISRG lit up Let's Encrypt.
But because the cert lifetime was 90 days, Bill gets told it's expiring at about the same time he's about to shove that "Actually automate certificates" work onto the back burner. Oh yeah, right, we should actually do that.
Now, it may be that at a startup that "Automation" is initially "Put it on Natalie the intern's TODO list every Monday" but by week six Natalie has learned about cron, or Windows event scheduling, or a dozen other ways to make stuff you always do regularly happen without actual work. Today I expect her LLM will tell her how to do it.
And Natalie is going to discover that some "permanent" solutions are more permanent than others, that you want monitoring and so on. But because Let's Encrypt is so popular unless you're doing something pretty esoteric you won't even be the first person to ask about it this month.