> Wait, then why don't they just switch over each component in turn? The "divide and conquer" debugging strategy.

Let's say the CPU is the actual issue, but the problem manifests itself in the memory module. You swap over to the backup memory module, and suddenly the problem vanishes!

Two months later, the problem manifests again. Identical presentation. This time, there is no backup to switch over to test.

You fly a Very Expensive Mission to the telescope only to find out the CPU was the issue, and if you had figured that out originally you wouldn't be up here with four memory modules.