ok I'll bite...Platform pedals have no mechanism for holding your feet to the pedal. If you put some kind of strap and toe cage assembly on platform pedals, it's called (I think ) toe clips. Clipless pedals, are pedals with much smaller platforms, but with a mechanism that allows your shoe to attach to the pedal that is compatible with the pedal, hence "clipless".

Why do we park on driveways and drive on parkways? Jumbo shrimp?