Widescreen masters do exist -- they've been used overseas and by some streaming services -- but I doubt CBS would spend the money to re-author the Blu-Rays in widescreen, when the previous versions would do.
Also, they look horrible, since all the live action footage has been cropped in them from their original aspect ratio (1.33:1).
This reminds me of the fiasco with "Kung Fu" on DVD some years back.
Would be cool to have a widescreen version in addition to my current full screen Blu-ray's.
Confidentially... Perhaps this is sacrilegious, but when I watch 4:3 shows, I use the "zoom" function on my HDTV to crop the top and bottom of the picture and fill the screen.

Kor