Why aren't Atlas and P-Body good enough for GLaDOS? [Spoilers]

Solution 1:

We don't exactly know what GLaDOS is testing for.

However, from the events of Portal 1, we can guess she's testing how efficient humans are in solving puzzles when under psychological stress.

And indeed, she did put a lot of stress on Chell, such as:

• Telling her she'd die if she doesn't solve the puzzle.
• Claiming the chamber would not be monitored.
• Forcing her to murder her only true friend.
• Having her run through a live fire course designed for military androids.

None of which would truly matter for computers such as Atlas or P-Body, since they ultimately aren't human. If they die, they could just be rebuilt.

In the end, all of this is just a guess and we still don't know what exactly GLaDOS is testing for.