Deleting questions defeats the purpose of the site, which was to be a repository for knowledge and experience. A place where people could find information they needed, or ask a question about it if it wasn't already posted here. Yahoo stopped allowing questions to be deleted many years ago, and Verizon Media (the company that now runs Yahoo) hasn't changed that.
Also, the ability to delete questions at will allows some site abusers a way to post serious violations (violence, porn, etc), and then remove them before they can be reported enough to get dropped. By disallowing users to remove their own questions after any answers have been posted, it adds a level of responsibility to the posting user...knowing that what they post may be on the site and available for all to see, for many years to come.