The stupid answer is that not everything that can be automated should be.

The real answer is of a more philosophical nature, if you manually had to check A, B, C... Z, then you will have a better understanding of the state of the system you work with . If something goes wrong, at least the bits you checked can be disregarded and free you to check other factors. What if your systems correctly report a faulty issue, yet your automatic checklist doesn't catch it?

Also, this manual checklist checks the operator.

You should be automating everything you can, but much care should be put into figuring out if you can actually automate a particular thing.

Automate away the process to deploy a new version of hn, what's the worst that can happen?

But don't automate the pre flight checklist, if something goes wrong while the plane is in the air, people are going to die.

I think a less verbose version of the above is that a human can detect a fault in a sensor, while a sensor can't detect it is faulty itself.