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 where it makes sense 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)?