You are correct. Credit is usually not enforced successfully. This is because many are not making the effort to enforce it successfully. This starts by adding the line in your contract stating that you or your firm receives credit when the rendering is published. This finishes with you placing a call or email to the websites that are using your image without credit. Usually these sites are very accommodating and will add your name.
Why do they do this... because this is standard practice in journalism. How often do you see a photo on a blog or in a magazine that has small print that read "Photo by ...." The image credit should be no different when it comes to renderings. In fact if you don't include/ask for the request for credit then often the architecture firm distributing the image will be given credit for the work in the medium where it is published.
When that big project you are working on goes live to websites, blogs, magazines, newspapers, etc.. you will want your name associated with that work.
(Evermotion is a different beast. Evermotion is a royalty free content provider which is very different from a studio that specializes in commissioned work. The business model of Evermotion has more in common with Getty Images than with a DBox/Neoscape or even a smaller studio.)