Because "sterile" exotics often have a seed production rate at >0% in the best scenarios.
Why introduce an invasive species into the environment when there are plenty of natives that will do the trick?
Stop being so lax with noxious weed contamination risks.
Please be aware of local native plant populations when using this concept. In much of the US some species of willow and buckthorn are invasive, can out-compete native plants, and can result in a drastic reduction in local biodiversity. I recommend Doug Tallamy's books Bringing Nature Home and The Nature of Oaks for anyone interested in native plant info. The writing is targeted at the US, but they're easy reads and have a lot of good info.