Why? It's not hard to do if you start a fresh project.
That being said, we do something similar where we require 80% coverage.
100% goes into "change detecting test" territory. There's also the time aspect: going from 0-70 is not hard, 70-100 is extremely time consuming, and often not worth the effort.
Monitoring is a way more efficient tool at catching issues.
We've found that with using Jest and just doing snapshots you can get to 70% without actually testing any of your others methods, hence the 80% coverage requirement.