Metal bodied planes are adjusted with thumb screws and levers - they just aren't accurate enough for fine work.

Re: a woooden body, there are a couple reasons it's preferable despite the maintenance - the biggest is that they can be adjusted to fit a specific blade and chip breaker. Since the blade and chip breaker are made by hand, you can't mass produce a body and still have the tool perform. The other big concerns are weight and economy. Metal bodies are way heavier. And if they break or are damaged, can't be easily fixed. A wood body can be made in a few minutes with materials that are usually found on hand.