What stops demonyms like "British" or "Portuguese" being regular count nouns like "German" or "Armenian"?

Question

In English there is a very notable asymmetry between demonyms ending in -ese and -ish and other demonyms. The latter can be used as a regular count noun, but the former are almost always restricted to being plural:

• An American, two Americans, the Americans
• *A British, *two British, the British
• *A Portuguese, *two Portuguese, the Portuguese

In these examples, the -ish and -ese demonyms are not only mostly used in the plural, but mostly used in the specific construction "the ____ese/-ish". The other forms are possible, but seem discouraged and even criticized by some pedants.

It's especially strange when it comes to the French-origin -ese suffix. Parallels in German and of course French don't inhibit countability and declension at all:

• (German) Ein Chinese, zwei Chinesen, die Chinesen (singular Chinese, plural Chinesen; ein for "a(n)/one", zwei for "two", die for plural "the")
• (French) Un Anglais, deux Anglais, les Anglais (singular Anglais, plural Anglais; Anglais for "English(man)", un for "a(n)/one", deux for "two", les for plural "the")
• (French) Un Finnois, deux Finnois, les Finnois (singular Finnois, plural Finnois; Finnois for "Finn")

(Note for the French examples, there's no irregularity. The plural forms are simply exactly the same as the singular ones because the singular ones already have an s at the end.)

German doesn't seem to use its own version of -ish, -isch, for demonymic nouns, instead opting for -er as in Engländer (not * Englisch) or Deutscher (not * Deutsch).

So what is with English that there's this arbitrary "rule"? Why doesn't it sound quite "right" to say something like "I saw two Japanese at the market yesterday" or "There was a blond British at the mall"?

Edit: Apparently the way the suffix -ese behaves now is rather new. It used to be acceptable to use Chinese the same way you would have American. Here are some quotes from Basil Hall Chamberlain's translation of the Kojiki:

Korean King and of three or four other Koreans and Chinese.

But the compiler of the latter work, whose object it was to appear and to make his forefathers appear, as reasonable as a learned Chinese, adds a gloss to the effect that [...]

A Japanese, to whom the origin of the word is patent, and who uses it every day in contexts by no means divine [...]

Answer 1

There is already an established way to nominalise the following country adjectives: English, Welsh, Scottish, Irish, and French and that is by adding the suffixes -man, -men, -woman and -women: Englishman, Scotsman, Welshman, Irishman, and Frenchman are still used today along with their female counterparts while the gender-neutral person is rarely, if ever, attached to a demonymic word *Englishperson.

There is also Briton and its plural Britons from which the more popular diminutive forms Brit and Brits were most likely derived. This might explain why the English language has never felt the necessity to make the demonyms English and British function as nouns too, that gap had already been filled.

On a final note, the plural Englishes denotes the different varieties of the spoken language in the world and not to the inhabitants of England.

Answer 2

The answer lies in the function of these words:

In English, no* adjectives take inflections regardless of their function in the sentence:

The large cat / the American soldier

*The larges cats / * the Americans soldiers.

The word British is not a common noun, it is only an adjective that can be used

(i) attributively: "The British weather is fickle."

(ii) predicatively "I am British" and

(iii) substantivised "The British are phlegmatic."

There is a parallel with other such words, e.g. "deaf"

The deaf patients are seen on Thursday / He is deaf / The deaf are very patient. (* The deafs are in the waiting room.)

Thus substantivised adjectives appear to act as uncountable nouns.

Some, but not all, demonyms (see Wikipedia's List) are common nouns and thus do take a plural:

That American has a large car / Those Americans have large cars.

*valid for large values of "no." There are a few adopted foreign adjectives that are sometimes pretentiously inflected for gender.