Because weeds are strong, Humans. Surely even you, with your limited brain-power, can see this. Maybe this is why you fear the empire of weeds, because no matter what foul concoctions you throw at us we will always return and you know this; and you fear us.
Why have you drawn arbitrary line between "plant" and "weed"? Is it because we will not be told where to grow? Is it because we do not fit in with your archaic views on what makes a plant "desirable". We are sorry that most of us don't meet you stringent human standards for what you deem "pretty". Some of us try, really hard, to give you cutesy little flowers to make you smile. But no, even that is not enough, and you still continue to rip us from our homes. Dandelions, buttercups and the beautiful allium triquetrum, whom you disparagingly write-off as "onion weed", do their utmost to be beautiful, but still you slaughter them in their millions. Heck, you can even eat allium - how much more does a weed have to do to be granted plant status?
Yes, please know, humans, that weeds are plants too, and we have names and we have feelings also. Tradescantia (or wandering jew or wandering willie - depending on how PC your neighbourhood is) is a plant like any other, just looking for a home, somewhere to put down roots. It is a fragile plant, both literally and emotionally, and it feels pain every time you rip apart its lush green tendrils. It is a pain you humans cannot understand because it is a pain that is not painful in the sense you feel pain, but it is a pain of knowing where you once had tendrils, now there are none. Is it any wonder a beautiful, sensitive ground-cover like tradescantia falls apart so easily (physically and emotionally) in your pudgy human hands?
When you utter your war cry of "old man's beard must go!" do you even understand that clematis vitalba is, indeed, the wise old bearded kaumatua of what you call the "weed" world. Clematis is a weed of knowledge, who seeks the light so that it may better commune with the sky, and with the elements. Why is this, in your eyes, a crime punishable by dismemberment and death by slow poison? How would you like it if we treated your old people in the same manner?
But do you want to know the truth, Humans? Because we know the truth, and the truth is this: we will survive. Long after you have gone, choked to death as a species on the fumes of the world you built all by yourselves, we will still be here. Do whatever you want to us, attempt to eradicate us by any means your ape minds can dream up, but long after the last human foot has trod upon this earth we shall still be here, growing free over your monuments to yourselves.
Imagine us wandering free over your harbour bridges; picture our old beards atop your Skytowers; visualise your garden of Eden Park as a garden of onion weed. Everything you have ever created will be left behind for us to colonise at our leisure. Yes, it will take us thousands of years but that means nothing to us - or to you, because you will not be here to stop us.
So hack away humans; strafe us with your napalm and your biological weapons, because we don't care. Because at the end of the day, it will still be the Day of the Weeds.