They were called floppy disks as compared to hard disk packs. The hard disk packs had their platters made of glass or something rigid. Floppy disks had platter

They were called floppy disks as compared to hard disk packs. The hard disk packs had their platters made of glass or something rigid. Floppy disks had platters/recording surfaces that were plastic, thus "floppy". Had nothing to do with the envelopes as such.