You’re right, they COULD letterbox Nolan’s movie. They won’t.

Nolan filmed in IMAX. It could just be reduced in size, as you suggested by letterboxing, for standard screens. But they won’t. They don’t want you experiencing the full size movie on a standard screen. They want you to go to IMAX because it costs more.

It’s not about the experience. It’s about money.

It’s no different than car manufacturers of the 1980s selling a base model vehicle with hand crank windows. They could have used the same assembly line to cheaply install machine powered windows for every vehicle of that model, but they intentionally did things to push customers towards more expensive models.