Learn Italian: Tu prepara la cena, intanto io pulisco. - You make dinner, meanwhile I will clean.

Because this is a command! For -are verbs like preparare, the informal imperative (command) form for tu ends in -a, giving us prepara. If it were just a statement of fact ('You are preparing'), it would be prepari.

This lesson is about the Futuro Anteriore. Why isn't it used here?

The Futuro Anteriore is for an action that finishes before another future action starts. The word intanto (meanwhile) tells us these actions happen at the exact same time. Since there is no chronological before-and-after, we don't use the Futuro Anteriore.

The translation says 'I will clean', but pulisco is in the present tense. Can I use the future tense here?

Yes, you could say io pulirò, but in Italian, the present tense is frequently used for future actions that are planned, immediate, or certain. In everyday conversation, io pulisco is very natural.

Just a quick refresher: why is there an -isc- in pulisco?