Broadly speaking, because they have to be able to handle a huge range of conditions. A Cessna 152 (a single-engine, two-seat, low-performance aircraft popular

Broadly speaking, because they have to be able to handle a huge range of conditions. A Cessna 152 (a single-engine, two-seat, low-performance aircraft popular for flight training) isn't very complex to operate. It can't fly at 30,000 feet, cross continents in a single flight, fly in icing conditions, land in zero visibility, safely take off or land in a crosswind that could be described as more than a stiff breeze, or carry much more than two people and a backpack. Airliners do all of those things. Military aircraft have similar performance demands, and people shooting at them. Automation can probably simplify everyday operation more than it has so far, but virtually every system that can be put on an aircraft has failed in flight at some point so pilots have to be able to quickly identify malfunctions and manually override a half-working system. For a recent example of what happens when that isn't the case, see the 737 Max.