It's an accessibility thing. Time travelers are covered under the ADA. Once a symbol reaches ubiquity, it can never be changed! Back in my day about 100k years ago, before I touched a mysterious stone in Scotland and found myself in the 21st century, we put an old sandal in front of the cave entrance if we needed privacy! Now it is a sock on a doorknob! 100k years from know, people will still be marking entrances with footwear for privacy.