They might give a very helpful answer.

They might give a very good answer, but so different from what you expected that it doesn't make sense to you.

They might give a very good answer, but their idea of cause and effect might be culturally so different form yours that you do not understand.

Some reasons are more historical, or more at assumptional, unconscious level that they cannot easily explain.

People often don't know the reason why:

• Understanding, using, and passing on that knowledge may have the job of designated people.
• They might never have considered that there needs to be a reason, because they have never considered an alternative to reality as they know it.