Why have backlit keyboards on laptops then?
Pretty much all keyboards come with LED backlighting these days. To not have them is actually not the norm.
There’s more of an argument to be made for not having backlit keyboards on laptops due to the size of the chassis, laptop battery life, etc. No such constraints exist with a stand alone keyboard, unless those constraints are purely artificial.
With Apple I think it might be two reasons.
I have both an Imac (with keyboard) and a Macbook Pro.
My Imac has white keys so I can always see the keys perfectly fine from the screen light even in a dark room. There really is no need for backlight in order for me to see the keys.
On top of that stand alone keyboards are mostly used in stationary set ups where most people either have their room light on or can just use a lamp.
With laptops you can be anywhere and might not have access to your own light and macbook pros have black keys which makes it harder to see the keys and generally smaller screens with less light to illuminate.
It's likely when Apple first made the magic keyboard they tested it, looked down and saw they could still see the keys even in a dark room and probably said leave it like that.
Now the space grey magic keyboards MIGHT be a different story, I don't have one so maybe you can see them fine, maybe not, but I doubt Apple was going to change the design for just those, especially since they don't even sell as many as the white. Plus like point 2, most people will either use them in lit rooms or can just get a lamp or maybe even have ample enough light from a big desktop screen. Plus these are bluetooth keyboards, you're going to drain the battery quicker for something that's not NEEDED. If I wasn't conscious of turning off my backlight I would be more annoyed by the shorter battery life than the non existent absolute need for a backlight.
Back lighting on stand alone keyboards just look and seem really cool. Of course I'd think that I'd want one if it's available but I've never used my Imac in a dark room and said "Damn I really NEED backlit keys right now" because I can always see the keys perfectly fine.