Expressing things as percentages was a late arrival; when until the mid 70s folks had to be able to cope with, and mentally manipulate vulgar fractions.
"four out of five" has the same number of syllables as "80%". "Three out of four" has even fewer syllables than 75%, for that matter. So I can see why those stuck around. Meanwhile "one out of two" is wordier than just "half". Are you saying British people still use it in such an inefficient case anyway?
