They are occupying the space left behind by human activity. Chemicals will only create more resistant strains just like it did with Amaranth. The reason these plants are successful is the horrendous landscaping traditions especially in new developments. The use of chemicals only reinforces these plants. Basically what's happening is these strains are put under so much pressure in the urban environment that they becomes "super-weeds" and then invade the rest of the landscape. It might be too late to do anything about it now.