It's like wondering why this grape doesn't taste like fried catfish. They're both food. Why don't they taste the same?
Width and height are meaningless for inline elements, but automatically changing the display would be more confusing, not less.
Really not sure what you're trying to get at there, obviously any element that displays will have a width and height. Maybe you meant a user specified width/height, but the entire point of my post is an inline-block is an inline element with a specifiable width and height. And we've always had the IMG tag, which is also an inline element with a specifiable width and height. The obvious and intuitive choice would have been to not put artificial limits on inline elements.
You could even do something crazy like `* {display: block;}` if you really want, and then go from there. Nobody can stop you. The freedom is yours.