Why is "a Japanese" offensive?

When talking about a person from Japan, why is it offensive to say "a Japanese" rather than "a Japanese person"?

The English language Wiktionary says

(person in or from Japan): The singular “a Japanese” is less common than locutions using the adjective, such as “a Japanese person”, and is considered potentially offensive by many speakers.

but doesn't explain why it is offensive.

Allwords.com says

Note: many people object to the usage of this sense in singular form, and it is now more frequent to see a person in or from Japan referred to by using the adjective Japanese. Rather than "a Japanese," you will frequently see "a Japanese person."''

but again, there's no explanation.

Answer

I found an answer in this article: 「私は日本人です」は、I am a Japanese. それとも"I am Japanese?

In English there is a distinction between nationalities that end in 'ian' like Canadian or Italian and those that end in 'ese' like Japanese or Burmese.

Those that have the 'ian' can say 'I am Canadian' or 'I am a Canadian' interchangeably, but usually in English the 'ese' ones don't use the 'a' when they're referring to people--unless it is being used as an adjective.

You could say "That is a Japanese person" (where Japanese is an adjective describing the word 'person')

This convention also applies to nationalities that end in 'ish' as well, as in 'I am British' or 'I am Scottish' --though Scotland is a different case because they could say, "I am Scottish" or "I am a Scot".

I don't really know why this convention exists, but saying 'I am a Chinese' to a native speaker would be very strange for them.