Perhaps because it isn't? The latter is written with an umlaut.
(Note that umlauts can actually be a distinguishing factor for very different meanings, e.g., "schwül"->humid vs "schwul"->gay. In the case of "uber", there is simply no such word in the German language; the preposition is "über".)
(Note also that in cases of typed German where there is not umlaut key available, the standard way to transcribe is to insert an "e" after the vowel. So you would write "ueber" for the German preposition.)