There are composites which are fiber based but those aren't labelled as steel toes, they aren't designed to shatter and it's not plastic.
But if you're an actual worker that relies on them to save your toes, CT has much worse puncture resistance than ST. So it's trivial to puncture with a framing nail gun or cut through it if you rest a board on your toe, for example.
They also shatter when something heavy falls on them. If the thing bounces and you're quick enough to pull your foot, then great. If not, the second bounce will be shattering your toes and your boot will be filled with fleshy composite soup that is impossible to save. A steel toe in that situation will just bend and odds are good that your foot will be salvageable.