Methods of joining rectangular sheet metal ducts used in the UK, as shown on http://www.ductstore.co.uk/acatalog/...r_ducting.html seem simpler than the method using drive cleats still common in the USA. (e.g. at 1:09 in https://www.youtube.com/watch?v=JIBIyf0JdPM ). What keeps the drive-cleat method in business?