Why say "four out of five dentists" instead of "80% of dentists"?
For all other fractions, I don't feel it is that odd to use X out of Y construct.
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.
It is also likely to be "one in two" rather than "one out of two", in speech "half", "three quarters", "seven eighths" etc is more likely.