They are built with extensive redundancy. Airbus A320s have 5 separate computer systems, programmed by 4 different teams, for aircraft control, of which you only need one to fly the plane.

See https://pdfs.semanticscholar.org/d4ac/17fcc0db3396a3219785bd...

It's a question of what design choices you make. For instance Boeing only has three in its comparable 777 fly by wire system.