grammatical number - Why do we use the singular in a comparative sentence?

In a comparative sentence, like the one below, we use the singular form in the second clause:

Rebecca is the most beautiful girl in the school. She is more beautiful than any other girl

and not:

*[...] She is more beautiful than any other girls.

Why do we use the singular and not the plural? Why is the second example not acceptable?

Answer

The "any other" construction takes the singular because the comparison is one-to-one.

She is more beautiful than any other girl.

is equivalent to:

She is more beautiful than any other individual girl.

You could express a similar sentiment with a construction that takes the plural "girls" like so:

She is more beautiful than other girls.

But this works because, in this case, the comparison is one-to-many.

In short: as an adjective in this case, 'any' can take the singular