It's always about spelling, eh?

Of the "common" forms of long and short vowels, the more open version is chosen to represent a tone in what would be a subjective choice... not always the *best sounding... or even clear in meaning

beet (long)preferable to bit (short)
blah (longer) " " black (short)
bottom (short o) but (short u)

Max - I've never heard U (long) substituting for I (short), it is most often E (long) that does that job (eg. sounding as sung "sa-lu-tees")