CPTX is the extension used for all Captivate projects, normal and responsive projects. Those 'files' are in reality zipped folders with a lot of subfolders and files, content which can be different depending on the type of project: one example is the used theme which is not the same for a responsive or a normal project. Only the Video Demo projects have a different extension: cpvc. But such a video project can be included as a cpvc slide in a cptx project. Captivate is not as simple as a Word or a PPT document, lot more complicated.
A responsive project is meant to respond to the device that will be used to view the course, that is the reason for the breakpoint views which can have a different layout. Example: for mobile you can choose to leave out some objects because the real estate for phone browser is much more limited. Flash player doesn't work since quite a while on mobile devices, that is why it has no sense to offer a publishing option to Flash at all for a responsive project. Both interacitve PDF and exe publishinbg options also lack for the same reasons, because both those formats are also Flash-based.