Regular people use keyboard covers?? I don't think I've ever seen that. I have only seen them on machines that are ruggedized for harsh environments or keyboards that are in public areas for public use.
FWIW, you can also see them used for switching to DVORAK and other alternative layouts.
And for adding a second writing script to a keyboard with Latin-only labels.
Put another way: I am absolutely certain that more users would get a Mac with a Dvorak layout by mistake and want to return it than would actually purchase and keep one.
Plus Apple already does this for 20(+?) languages, some of which have difference shapes, which for a macbook means different frame milling. One more letter arrangement isn't much.
Plus we're overestimating the number of users of alternative keyboard users in the world. There's maybe a few hundred thousands of us. Compared to the millions of speakers these 20+ languages have.