That's actually not the case. If you order green electricity, the power company will buy certificates from Norway, for instance. The renewables never leave Norway and the power company basically greenwashes their nuclear/coal. All buying certificates does is guarantee that somewhere in the world, an equivalent amount of renewable energy is being produced and consumed. (Just read the small print.)
This is a huge flaw in the current system, where the certificates are completely separate from the physical power. This means that you don't get the needed economic pressure towards renewables where they are needed. For the most part, I consider the current system so deeply flawed that it amounts to fraud. It misleads consumers into believing they are receiving renewable energy, when in fact, they are not. The certificate system is probably negative for the environment in total, because it gives people a false sense of being green. This means that to some degree, you can expect that people will continue to use nuclear/coal, even when they could have installed solar, at maybe only slightly higher cost.