Possibly not. I suspect a lot of them are just a password with a changing number in them (which is what usually happens with a forced frequent change). Other similar sites seem to manage without password changes. Just because it appears to work doesn't mean that something less "annoying" to users wouldn't work equally well.
Code:https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/458857/Password_guidance_-_simplifying_your_approach.pdf