Do you mean delete their questions, their answers, or both? Can you give examples of when the facilty would be useful?
If you make an error in a question you can close the question.
It is an answerbank so if questions and answers were deleted the bank of answers would be reduced, which goes against the purpose of the site.
I have made errors when typing questions and responses (eg adding a link incorrectly, accidentally deleting some text and some spelling howlers) and sometimes wish I could edit existing postings of mine, but clearly that would make it difficult to follow a thread and would encourage some trolls to type abuse then remove it.
Worth a debate though.