Because tests only tell you part of the story, they don't tell you much about the physical condition of the installation. Tests don't tell you how well installed something is, how many joints are in a cable, whether the cable is the right size. Tests can't tell you that the wrong type of cable is installed outdoors, the cable isn't rated for the temperature it's exposed to.
There are many situations where something could test out perfectly but be in desperate need of being replaced.
To give some extreme examples:
A circuit wired in un-enclosed single insulated conductors pulled through joists, chased into walls etc could easily pass all tests but be completely unacceptable.
Bare conductors clipped to a wall could pass all tests.
A 20mtr long cable with 7 joints in it hanging in free-air could pass all tests.