Learn Finnish: Nämä kalliit vaatteet ovat nahkaa ja silkkiä. - These expensive clothes are leather and silk.

Because they are uncountable materials. When you say what material a subject consists of using the verb olla (to be), the material is put into the partitive case. Nahka (leather) becomes nahkaa and silkki (silk) becomes silkkiä.

Why does kalliit end in -t when its dictionary form is kallis?

In Finnish, adjectives must match the noun they describe. Because the noun vaatteet (clothes) is in the plural (indicated by the -t ending), the adjective must also take the plural -t ending. Notice how kallis changes its stem to kallii- before adding the -t.

Is nämä the plural form of tämä (this)?

Yes! Just like in English where 'this' becomes 'these', the Finnish word tämä (this) becomes nämä (these) to perfectly match the plural noun vaatteet.

Since this lesson is about the elative plural (-ista/-istä), why aren't any words in this sentence using it?

This sentence uses the partitive singular for its materials (nahkaa, silkkiä) because they are uncountable mass nouns. However, if you wanted to use the elative plural to say something was made 'out of' these specific items, you would use it on the clothes themselves: Nämä on tehty kalliista vaatteista (These are made out of expensive clothes).