If I understand it correctly, "basic" Linux has a version. You can assume that a certain version will have at least a certain version of various pieces.
With Android, there are not only more versions, different hardware manufacturers can (and do) decide to include or exclude whatever the hell they want. Not to mention* each device will be tweaked to assume a particular aspect ratio, and any app expecting otherwise will be borked.
That about it? Or is there more?
* Doesn't that phrase virtually always mean that you're just about to mention?