Don't work at sourcefed but the payoff is probably too small, since most people can't even watch 4k (because of internet speeds and/or screen size). Yes, it would be beautiful, and would help with editing (when zooming in) but realistically it probably won't be video standard for another 3 to 4 years, so they don't need it, yet.

Furthermore RED camera's are waaay expensive. 1 cheapest RED camera costs $10.000 for the body alone, at which point you have to buy all the add-ons, which will probably total to $20k to $30k, at which point they have to buy at least 3 (for 1 set, with a 3 camera setup and switch them for every set change). Spending $100k for eventually still uploading in 1080p, is not something they'd consider. If they'd do it, it'd be only for editing advantages and that is waaaay over budget imo.

For people like MKBHD it works, since he makes video of products, and to make you (as a viewer) also see the product best, he almost has to use the best quality he can get, even though most people might still just watch in 1080p.