It's about eliminating cognitive load when reading and/or modifying source later. The more code you can eliminate from what is being looked at, the easier it is for the reader to understand the purpose of the function and what it is accomplishing.

At some point, someone is going to need to change your code. When that happens, it is easier to 'surgically' modify a single method that does one simple thing, than adjust one tiny piece of a much larger function. Even though the change might be exactly the same, it will be easier to understand which callers are affected by the change so you can detect the impact of your modification.

It is a guideline, not a rule. You'll (probably frequently) have methods that logically make sense to keep longer, but if you're writing and find yourself creeping past that, it is a good indicator you might want to review the function and make sure you're not inlining something that might make sense as its own function.