From another perspective, a floppy is also secure because it is a completely dumb storage device: it's just a piece of magnetic media in a plastic case. The drive is equally dumb, and the controller IC is usually a hard-wired logic array. The format has been around for a long time and publicly documented in standards. Contrast this with USB drives which have their own firmware and microcontrollers that can hide maliciousness, or even CD/DVD drives that also have modifiable firmware.

Someone could intercept a floppy and read/write its data, but putting malware on it and getting the receiver to execute it unnoticed is going to be much harder than with a USB drive. Putting a floppy into a drive literally does not cause any code to execute - it's only when commanded to, i.e. when the drive is accessed, that things start happening. There's no AutoPlay for floppies. Contrast this with the whole USB enumeration process.