You can copy them, but with high level of difficulty as compared to copying ps1 or ps2 games.
A lot of Dreamcast games are mixmode (data + audio) and with DC they go a step further by using special cds they make copying harder. If you look at a DC game you should of noticed that there are some differences with them compared to any other type of CD. I believe they call them GCD. Some sites out there have been able to correctly copy these cds and then patch them to work properly using regular cds.
So there is a lot more to it than just making an image from the source and duplicating it. This is why I pointed you to gamecopyworld.com so that you can read more into it.