Simply because you didn't define a step, as seen in these docs
step
Optional. Extended slice syntax. Step value of the slice. Defaults to 1.
So because you don't have a negative step to tell it to step backwards, it doesn't
Additionally -5
refers to the 5th element from the end, which is greater than 0 so because its already at an index greater than its end destination, there is nothing to do.
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