This is the best answer. It's not because computers aren't perfect or people don't trust computers. It's because computers cannot account for all possibilities that could happen. Ask any pilot how much decision-making they do on a day-to-day basis. In an emergency, you need a human brain to be able to weigh the different options and take into account the intricacies of that specific situation. Just look at QF32, an extremely high-tech aircraft where the crew decisions are what saved the plane. Your engine has exploded and you've lost some hydraulics, do you turn back and land at your original airport, hoping you don't lose further control, or do you land at a closer one where the runway might be too short? The Wikipedia article gives a good example of the kinds of decisions they had to make based on what the aircraft computers were telling them:

The First Officer (FO) and Supervising Check Captain (SCC) then input the plane's status to the landing distance performance application (LDPA) for a landing 50 tonnes over maximum landing weight at Changi.[10] Based on these inputs the LDPA could not calculate a landing distance. After discussion the crew elected to remove inputs related to a wet runway, in the knowledge that the runway was dry. The LDPA then returned the information that the landing was feasible with 100 metres of runway remaining.[12] The flight then returned to Singapore Changi Airport, landing safely after the crew extended the landing gear by a gravity drop emergency extension system. Source