Learn English – Articles for words that function as both a verb or a noun

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.

Well, first of all, we don't need an article before any noun. I can say:

I like dogs.

As to your question of why, the answer is, "because that is how English works".

The articles perform a discourse function, by indicating new and old information. They often evolve from demonstratives (e.g. "this" and "that"). They evolve independently in unrelated languages. Normally, once an article system becomes a part of a language, it is an all-or-nothing thing. This is not unlike a verbal inflection system, a gender system, or a case system; once the system is in place, it is not optional.

Aside from this, there isn't really a more concrete reason for "why" English works this way.