Why are words such as "that" and "those" not considered articles?

Solution 1:

Eldros got it right in his comment. This, that, these, and those are derived from various inflections of the demonstratives þes and þæt in Old English, which were used both as adjectives and as pronouns, just like their modern forms.

You could argue that since demonstrative adjectives are never used with an article (*the this hat), they are roughly equivalent to articles, but articles specify definiteness while demonstratives specify location, which may imply definiteness, but only incidentally.

For instance, I can say "those apples" whether or not I've already mentioned the apples in question. If the apples are definite, then a demonstrative simply provides emphasis or clarification:

I bought some apples last week. Those apples were really good.

But if the apples are indefinite, it gives them a location and makes them definite as a side-effect:

Hand me those apples next to you.

Solution 2:

"That" and "those" (in their capacity as pronouns) are not "indefinite pronouns", but "demonstrative pronouns". The fact that they have a category of pronouns other than the "indefinite" category does, I think, suggest that they are indeed definite! And when they become adjectively, I doubt that they lose their definiteness in the process.

In languages having a definite article, the lack of an article specifically indicates that the noun is indefinite.

"The children run" has a definite article. "Children like to run" is an example of an indefinite noun. "My children like to run"? I think we're in the realms of the definite again. I suspect "the lack of an article or another part of speech in place of the article" would be a better, um, definition of indefiniteness!