Proper PoE sources have active per port current monitoring and will disable the PoE power in case of an over current event.
You can be over the thermal capacity of the cable without having too much draw on the port.
So the situation would be: you create a setup, buy devices and then they randomly shutdown as they pull too much current? How is that better than having a well defined standard that ensures compatibility?
The device first negotiates a certain current capability. If a device explicitly asks for 15W and then goes on to draw 60W, you can hardly call it a "random" shutdown: it is clearly misbehaving, so it is best to shut it down to prevent further damage.