We already have a number of empty templates, they're labelled "without content, design only". You can add multiple modules to your pages (text, images, etc.). You can also add a source code module where you can insert your own code.
If we “opened” the entire page to code, we would not be able to customize the modules to each possible code variation. A great number of errors could arise and websites would not display as desired. This is why importing html code for an entire page will not be supported in inCMS.