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?
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.