What is the difference between 'had a bath' and 'was in the bath'?

Please could you explain in simple English why using “had a bath” as apposed to “was in the bath” in the below answer to the question is incorrect:
“Why didn’t you answer the telephone?”
“Because I had a bath.”
Thank you

The answer could be either: because I was in the bath or because I was having a bath.

Thank you for the response. Is there any way you could explain in simple terms why the original answer was incorrect?