It's not that simple. You need a format that can represent any kind of document. Good luck trying to convert a powerpoint with complex shapes and background images into a RTF. There are only two formats that can really do that, while being viewed inside a browser - swf and images.
Does anyone want a PowerPoint with complex shapes and background images?
Nobody is stopping you from using latex,beamer,and emacs and taking a weekend to finish your slides. But if you want something done in fifteen minutes that looks good, use powerpoint.
I am faster using TeX's beamer classes than fighting Powerpoint's UI.
RTF is a good format for editable documents, but it's not such a good format for documents that need to look exactly the same on all platforms. The format that PDF actually reinvents is Postscript.