Honest reply: because the infrastructure isn't ready to support 1-day certificates yet. If your cert is only valid for one day, and renewal fails on a Saturday, then your site is unusable until you get back to work on Monday and do something to fix it. There are things that can be done to mitigate this risk, like using an ACME client which supports fallback between multiple CAs, but the vast majority of sites out there today simply aren't set up to handle that yet.

The point of the CA/BF settling on 47-day certs is yes, to strongly push automation, but also to still allow time for manual intervention when automation fails.