Why not ? like i said everything that makes life easier is a good thing , there are times when backlit buttons may even be faster than the screen - hard to know as I have never had the chance to use both.
You are also trying to make out its about learning buttons, i have used a pc keyboard for 40 years but will still make the odd mistake if i try typing in the dark , in pressured time sensitive situations like weddings there is a lot going on and no matter how well you know the cameras its the little things that help to make sure you can concentrate on whats important.
When you drive your car in the dark you will note that the buttons on your dash are backlit - so you can find them easily and quickly - its no different.
Well... can you give me example on what settings you'd change where the buttons would help? For typical shooting, you basically need:
-shutter
-back focus button (for those who use it)
-shutter (usually dial)
-aperture (usually dial)
-ISO (usually button+dial)
-AF mode (usually button+dial)
-AF point selection (usually joystick)
All those I can hit blindfolded on both of my bodies even they are different. And I'd say anyone shooting weddings should be able to do those at least without looking. Beyond those, I'd be anyway using the screen menus with either dial+joystick combo, or touch screen, so the button lights wouldn't matter.
So can you give example what buttons you might need in hectic situation and it's not one of those listed above? Like I said, I believe there's such cases but I can't think of any as I have not encountered such.
As for "why not". Added parts is one more point for failure. Tiny added cost, and having even small LED per button plus the circuitry to control them and supply power to, adds on the volume/space too. The cameras are already crowded inside.
The car example is not valid comparison either. On camera you hand is basically resting on the buttons and assuming you know the buttons, you can't really miss. On car you reach out arms-length so if you're not looking the chance of missing is high. Also, I drive Tesla so there's not one single button, only touch panel
Happy New Year! More shooting, less fighting