Because fedoras used to be a staple of the "put together" look your grandfather probably wore, a step below a suit in formality with slacks and a wool coat. Most suits aren't available in tarp sizes (or at least the one that are, cost a lot) so death-fats will usually wear a casual button down shirt (usually with permanent food stains and some form of print) plus a fedora to "dress up" and use as "classy" formal wear.