This is why 5 why's is (or should) be paired with something like fishbone diagrams, or why tree's what ever to reinforce that it's not sufficient to regress along a single line - you must try to build out your causal tree out to 5 layers where possible.

Ultimately 5 why's is an arbitrary guideline that balances trying to keep you from being complacent in your analysis, with the need to actually like... stop and make a decision.