If you consider that a pixel is a container for RGB value, then the length and area collapse into simple storage requirement.

When an image is some px wide and some px high, it provides a shape (ratio between them), because a camera of 12mega px isn't 1px x 12M px in shape, but it does occupy 12M storage units nevertheless.

I agree though that like many IT related terms, their usage is twisted and simplified to amplify the buzz. Because it is more marketing friendly to put a bigger number than exact dimensions and proper terminology.

Is there any context that where it makes sense to to have fractional pixels? Like millipixels, micropixels, nanopixels, etc.? There is of course sub-pixel text rendering; does anyone refer to numerical fractional pixels in that field (333.33 mpx)?