there is lots added to 1. hardware and 2. firmware and 3. costs to add video recording.
Anything from different sensor design to (larger) heatsinks and bodies, additional control points (eg an utterly senseless mono-functional marked-in-red "record video button") to audio stuff like amps, stereo speakers, plus all sorts of connectors (+ cost to weather-seal them all), all the way to codecs/firmware stuff (does not come NOT free of charge!), sucking up battery and CPU cycles ... for something the majority of stills camera buyers never use or need. Plus a lot of clutter i menus. Thanks, but no thanks. Just make some pure stills cameras and see how well they'll sell and how few hybrid cameras will be bought, once dual use will cost some extra money rather than being demanded as free lunch by a few free-riders.
If we're talking mirrorless, which honestly is where everything is going, the sensor is continually reading what is coming through the lens at all times whether you're doing videos or stills. So, I don't think your idea that a camera that does video needs a bigger heatsink holds much water. All mirrorless cameras are essentially doing video all the time, whether you're actually recording it, or just doing stills shooting.
Again, I don't have my camera with me to confirm, but I think the video record button on the EOS R can be mapped to a variety of stills functions. So I guess just put some black tape over it and pretend it's a custom button if having a red button originally intended for video recording bugs you that much?
The idea that just having video functions in a camera is somehow sucking up processing power or battery life even when you aren't actually in video mode sounds like some serious tinfoil hat stuff to me. I'm not a software developer, but I would think that the code within a camera would be horribly inefficient if a mode you weren't even using were using battery power or processing power at all times. If this is actually somehow the case, I think the amount of battery or processing video functionality uses in stills mode is infinitesimally small.
If the menus bother you that much, set up your own custom menu... All the stills functions you could possibly want, no video stuff. Problem solved. Besides, don't the menus change depending on if you're in video or stills mode? I thought some of the video options disappear when you're not in video mode.
I will admit that you could get rid of a few connections and the microphone on a stills only camera. But you still need a speaker for focus confirmation beep.
But, the development costs to make a completely separate body that just deletes connections and the microphone, as well as unique firmware without video functions, would result in this mythical "stills only" camera costing just as much as the video camera. A microphone and some connectors are not adding that much to the cost of a camera. And if you factor in the smaller volume that such a camera would sell, it's unlikely that they would be able to sell a stills only camera any cheaper than one with video. That's the whole point of making one camera that can be used for both, you're able to sell it more cheaply because you will sell a greater volume since people who want to do stills and video will both buy the same camera.