For the same reasons as forcing companies to do it.
1. Revocation is a clusterfuck. Microsoft is currently failing to revoke tens of thousands of defective certificates for over seven months (the Baseline Requirements state that they should have been revoked within five days). Entrust was distrusted over repeated failures to revoke. Many TLS clients don't even bother to check revocation lists, or if they do they do it in a "fail-open" manner where inability to load the list does not prevent access which largely defeats the purpose. Short certificate lifetimes make revocation less important as both defective and compromised certificates age out rapidly, but without automation any meaningful reduction in lifetime directly translates to an increase in required effort which makes it a balancing act. With automation, however, reducing lifetimes is effectively free.
2. Automation is mostly a one-time cost. Manual renewal is a repeating cost. I started using LE to add HTTPS to my personal site when it first became available to the public in 2016 and then started using it for my work systems when our GoDaddy certs came up for renewal a bit less than a year later. Since then out of roughly 50 systems pulling around 70 certs I've had to "babysit" two of them once each, both because they were using wildcard certs which I was a very early adopter of and something about how I had set them up was technically wrong but worked in the initial version. Compare this to the "old world" where every couple of years I had to generally engage vendor support on both our server platforms and our CA because it had been long enough that things had changed on both sides so doing things manually required learning new things. Mandating short lifetimes is good for everyone. This is part of why LE has used a short lifetime since the beginning, to strongly encourage people to not try to do it without automation.
3. It's super easy. Lots of platforms have ACME support built in. For those that don't, packages like acme.sh are damn close to universal. If your system is able to expose a web server on port 80 it's basically trivial. If it's not, it's slightly harder. There's just not a good reason not to do it other than stubborn insistence. "Handcrafted TLS Certificates" just don't matter.