Because computers go wrong regularly
In the past two weeks I've had a hydraulic leak I spotted on a walkaround which didn't show up as low quantities or warnings on the flight deck. A computer would have taken off and then suffered a hydraulics failure - a portentially big issue when airborne. I spotted the problem on the ground before the computers even realised anything was wrong...
In a second incident we had an AHARS failure - essentially one of the the computers that drives the on board speed/pressure altimeter etc displays. What would the computer do when a part of itself fails? I know what we did - no problem.
I say sack O'Leary and replace him with a gerbil as Loughborough University did for the president of their student union some years ago - you'd save millions. Two channels 'yes' and 'no' would allow the gerbil to make all the same deicisions O'Leary does - I mean isn't executive management nothing more than a series of yes/no decisions???
I rest my case.
Desk-pilot