Learn Polish: Odkurzacze z plastiku czyści się szybko. - Plastic vacuum cleaners are quick to clean.

This is because czyści się is an impersonal construction (translating roughly to 'one cleans'). In this structure, the verb is always locked in the 3rd person singular (or neuter singular in the past tense). Odkurzacze isn't the subject of the sentence; it's the direct object being acted upon.

If odkurzacze is the direct object, shouldn't it be in the accusative case?

It actually is in the accusative case! For inanimate objects like vacuum cleaners, the accusative plural form is identical to the nominative plural. So even though it looks like a subject, it is grammatically the object of the verb.

Could I use the plural verb and say odkurzacze czyszczą się szybko?

Yes, you could. That creates a true 'middle voice' where the vacuum cleaners act as the grammatical subject undergoing the action. However, the impersonal form used here (czyści się + accusative object) is highly idiomatic and often preferred in Polish for general statements about how easily or quickly a task can be done.

Why does the sentence use z plastiku instead of the adjective plastikowe?

Both odkurzacze z plastiku and plastikowe odkurzacze are completely correct and mean the same thing. Using z followed by a material in the genitive case (plastiku) is a very common and natural alternative to using an adjective to describe what something is made of.