They do, most of the time. For example if you upload an heic file from an iPhone to a file input on a website that doesn't accept heic files, it'll upload a jpeg.

Apple can't see or control all the different ways of transferring files, though, so in practice it still causes problems sometimes.

The strange thing is that some Android phones also save photos as heic files and make no attempt to convert them, so I still had to add logic to my websites to convert them myself.