If you build a bridge, the engineers aren't the one doing the welding and crane operation and bolts and digging holes and whatnot.

They're the ones checking that work matches the plan.

Come on, now. The human writes the plan up front, which includes guidance on testing strategy, classes of tests, particular test cases to cover, etc. And just like normal, of course you don't just ship the code without doing manual verification, code review, auditing the test cases, and all the rest.