Electric arc furnaces, like aluminium smelters, typically exist at large scale in locations where there is a plentiful supply of cheap electricity, generally in the vicinity of hydro plants. There are places where it makes sense to generate and sell electricity cheaply to a local industrial customer than to transmit and distribute it in quantity to the wider market who'll pay the usual market price. There is a limited quantity of such cheap locally-available electricity.
More generally, for most of us in the wider world, cheap renewable electricity is only available at times of plentiful wind and/or high angle sunshine. Means that if you want to run on cheap renewable electricity, then you have to limit your operations to the times when it is available, be content to run your furnace for only a limited fraction of the time, and be ready to run it when the cheap stuff comes, whenever that might be.
If you want to run your furnace instead in a predictable fashion, and/or for a large fraction of the time, then that would be reliable electricity you need. Reliable electricity is more expensive than cheap renewable electricity. Reliable renewable electricity is, for most of us, very expensive indeed, and close to unavailable just now. The so-called 100% renewable electricity that electricity suppliers are, astonishingly, permitted to describe as such, is only 100% renewable electricity, subject, as Private Eye recently put it, to a highly contorted definition. It ceases to be a useful definition as we decarbonise further, if indeed it was ever a useful definition. The proper stuff is a lot more expensive, at least until there is major technical progress making major reductions in the costs of various technologies.