The specification of such a high voltage is to be able to send much power with low current.

But the cable length doesn't really matter - at the lengths that CAT-5 is specified to work with the network, the voltage loss isn't too important.

The important thing here is that you can't send more power just because the cable is shorter. All it means is that you will get a slightly lower voltage loss in the cable. But every meter of the cable will still see the same current.

Next thing - if you send 3.3V directly over the cable, it will not be PoE. And you still must keep the current low, so with less than a tenth of the voltage you could send less than a tenth of the power for the same current.