I have finished washing my hair and it is still wet. Why is my hair wet?
Because I have washed it. (focus is on the final result)
Because I have been washing it. (focus is on the action)
To me they are both possible because the recently finished action leads to the result = wet hair.
So what do you think? Which one is preferable?
Both are possible, with very little difference in meaning.
It is also possible to use simple past
Because I washed it (five minutes ago); I just washed it.
or past continuous
Because I was washing it.
All possible, all correct. Not much difference in meaning.
I believe you understand the difference between the two verb forms quite well.
I think for washing one's hair, a focus on the action of washing one's hair would be rather unusual. So the most natural conversation would be:
Why is your hair wet?
Because I've washed it.
But for other scenarios, the focus on the action would be completely natural. So the following conversation would usually use the past continuous tense:
Why are your shoes all muddy?
Because I've been walking in the woods.
Maybe the reason for the difference is that most people wash their hair in order to get clean hair, while many people walk in the woods because they enjoy the walk.
Both verb forms are grammatical for both situations, and probably wouldn't strike a native English speaker hearing them as unusual.
I don't believe the set-up here. Who wants to know? If it is the person asking the question, they would know whether or not they have been washing their hair recently. Otherwise, the question reads like some kind of riddle, or possibly a rhetorical question.
If the real intended question here is: "Why is your hair wet?" then "Because I have washed it" without further context might possibly be OK as an answer, but only because the listener will mentally interpolate the word "just". They would need to make such an interpolation because the statement "I have washed my hair" is true of pretty much every human being on the planet who has hair to wash. The statement therefore contains literally no information without further context.
It would be easy to think of circumstances in which that truthful answer might not in fact account for the hair being wet now: "I was just skinny-dipping in the pool with your wife", for example.
Consider, for example, both of the following statements that are in fact true of me:
There is a difference in meaning between the two formulations in the OP's question. That difference might not matter in some circumstances but would in others.
