[thwarted]

Why say "four out of five dentists" instead of "80% of dentists"?

[albert_e]

Seems a bit needlessly long when employed for a concept more mundanely called "half". A lay person uses half more often, I would imagine.

For all other fractions, I don't feel it is that odd to use X out of Y construct.

[dfawcus]

Because that is just what we do.

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.

[absolutelastone]

"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?

[dfawcus]

The media does when writing such things up, stylistic choices.

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.