The days of film
In the days of film (remember them?), what a camera didn’t do was contribute to the “look” of the image, or, perhaps, only peripherally. The actual quality of the picture came from the film itself, which was not part of the camera. That’s the biggest change of all in the new digital paradigm: that the camera manufacturers are now responsible for this look, where “look” includes just about everything to do with the image: contrast and dynamic range, colour, sensitivity, highlight response and a myriad other parameters that can affect the output from the device. It’s only the lens-mount and a few other light-domain aspects of the camera (such as whether it has a mechanical shutter or not) that can be extrapolated from previous film-based designs.
The biggest common link between these two eras? Ergonomics. The human form hasn’t changed as we’ve gone digital, so the ergonomic needs of camera operators are approximately the same. So it’s reasonable to think that older camera vendors will know a thing or two about this.
But most of the skills you need for the digital domain are... digital. If you’re going to build a digital cinema camera, then you need to know how to take the raw image off the sensor, and process it so that you get the best possible looking output. Of course a lot of this is done off-board in a raw processing workflow, but there’s stuff going on inside the camera that only those in the deepest inner circles know about. So at the very least, if you’re putting together a camera “off the shelf” you need to recruit some image processing experts as well.
One source of expertise is just “out there”, on the internet. It’s a model that can work, sometimes, with software. Open source development depends on the willingness and capabilities of those taking part, but it can lead to very substantial achievements like Blender. There are even tentative steps into Open Source hardware with Arduino and the camera project Apertus.
Even if you can build a camera - and the chances are that if you know what you’re doing, you will be able to; you still have be able to run a business. And that’s where the likes of Blackmagic and RED have a distinct advantage: They’re young and immensely flexible, proven in the world of high-tech business, and able to turn ideas into reality in a short time. And they're able to market them extremely well to an eager audience.